30th September 1961:

“John and I went on a trip for his twenty-first birthday. John was from a very middle-class family, which really impressed me because everyone else was from working-class families. To us John was upper class. His relatives were teachers, dentists, even someone up in Edinburgh in the BBC. It’s ironic, he was always very ‘fuck you!’ and he wrote the song ‘Working Class Hero’ – in fact, he wasn’t at all working class. Anyway, one of John’s relatives gave him £100 for his birthday. A hundred smackers in your hand! That was a real windfall. None of us could believe it. To this day if you gave me £100 I would be impressed. And I was his mate, enough said? ‘Let’s go on holiday.’ – ‘You mean me too? With the hundred quid? Great! I’m part of this windfall.’” - Paul McCartney, Anthology

“We planned to hitchhike to Spain. I had done a spot of hitchhiking with George and we knew you had to have a gimmick; we had been turned down so often and we’d seen that guys that had a gimmick (like a Union Jack round them) had always got the lifts. So I said to John, ‘Let’s get a couple of bowler hats.’ It was showbiz creeping in. We still had our leather jackets and drainpipes – we were too proud of them not to wear them, in case we met a girl; and if we did meet a girl, off would come the bowlers. But for lifts we would put the bowlers on. Two guys in bowler hats – a lorry would stop! Sense of Humour. This, and the train, is how we got to Paris." - Paul McCartney, Anthology

“And Paul and I also did the same thing, once. We just cancelled. We’d made it, in Liverpool. We were making good money, for those days. I can’t remember what it was – maybe a couple of hundred dollars a week – but enough that you’d have a little extra. You’d have it in your back pocket. And Paul and I just— A relative of mine gave me a hundred pounds, for my birthday, which I’d never seen that much money in me life. Paul and I just canceled all the engagements, and left for Paris… And George was furious, because he needed the money – to work, you know. But that was another time when the group was in debate as whether it would exist or not.”  - John Lennon, 1976, an interview with Elliot Mintz

“Last night I heard that John and Paul have gone to Paris to play together – in other words, the band has broken up! It sounds mad to me, I don’t believe it…” - Stuart Sutcliffe, Anthology

"They were brothers. They were the Nerk Twins, and now they were taking a break from the Beatles and going off to Spain. En route, they’d stop a day or two in Paris, to size up the Brigittes, check out the kind of clothes Jurgen Vollmer wore, and perhaps see Jurgen himself, if he was around. [Johnny] Gustafson happened to bump into them the day they left, Saturday 30 September. “They both had bowler hats on, with the usual leather jackets and jeans. They said they were off to Paris, so I walked down to Lime Street station with them and watched them go. They were an incredible pair: always great fun, irreverent, and so close.” - Mark Lewisohn, All These Years: Volume One

“We’d never been there before. We were a bit tired so we checked into a little hotel for the night, intending to go off hitchhiking the next morning. Of course, it was too nice a bed after having hitched so we said, ‘We’ll stay a little longer,’ then we thought, ‘God, Spain is a long way, and we’d have to work to get down there.’ We ended up staying the week in Paris – John was funding it all with his hundred quid.

We would walk miles from our hotel; you do in Paris. We’d go to a place near the Avenue des Anglais and we’d sit in the bars, looking good. I still have some classic photos from there. Linda loves one where I am sitting in a gendarme’s mac as a cape and John has got his glasses on askew and his trousers down revealing a bit of Y-front. The photographs are so beautiful, we’re really hamming it up. We’re looking at the camera like, ‘Hey, we are artsy guys, in a café: this is us in Paris,’ and we felt like that.

We went up to Montmartre because of all the artists, and the Folies Bergères, and we saw guys walking around in short leather jackets and very wide pantaloons. Talk about fashion! This was going to kill them when we got back. This was totally happening. They were tight to the knee and then they flared out; they must have been about fifty inches around the bottom and our drainpipe trousers were something like fifteen or sixteen inches. We saw these trousers and said, ‘Excusez-moi, Monsieur, où did you get them?’ It was a cheap little rack down the street so we bought a pair each, went back to the hotel, put them on, went out on the street – and we couldn’t handle it: ‘Do your feet feel like they are flapping? Feel more comfortable in me drainies, don’t you?’ So it was back to the hotel at a run, needle and cotton out and we took them in to a nice sixteen with which we were quite happy. And then we met Jürgen Vollmer on the street. He was still taking pictures." - Paul McCartney, Anthology

“Jürgen had a flattened-down hairstyle with a fringe in the front, which we rather took to. We went over to his place and there and then he cut – hacked would be a better word – our hair into the same style.” - John Lennon, 1963

Interviewer: I heard you took a trip to Spain before once, didn’t you? On Holiday? Paul: I didn’t go to Spain, no. I tried once to make Spain but… and John and I were gonna hitchhike. We hitchhiked down from Liverpool… We didn’t hitchhike. No, we got the train down from Liverpool ‘cause we thought we won’t hitchhike down the first bit. And we got the boat over to Paris. Then we got the train into Paris ‘cause we thought: “Well, it’ll be too hard to get a hitch here”. And we just stayed in Paris all week. And eventually… I mean, all the time trying to get out of Paris and make Spain! We never made it, we just flew home at the end. What a lazy hitchhiking Holiday!

“The thing was all the kissing and holding that was going on in Paris. And it was so romantic just to be there and see them even though I was 21 and sort of not romantic. But I really loved it, the way the people would just stand under a tree kissing. And they weren’t not mauling at each other, they were just kissing.” - John Lennon

"John’s 21st birthday was a month away, and he knew he was getting money — 100 pounds cash, more than he or Paul had ever seen in their lives. (…) Bob Wooler was party to their planning, and fought with them:

They were bored, and decided they would go away for a month. I thought this was disastrous because they would be away from the scene too long and lose their fans, Fans were very capricious: they moved from one group to another. And anyway, what about the other two members, George Harrison and Pete Best?. What about them, what do they do? We argued a lot about this — we argued in the back room of the grapes pub to a large extent —- and they said ‘Well, we’ll go away for a fortnight only’

(…) Equally, the promoters who paid the Beatles over-the-odds to present them every week had to “lump it” (….). To a man, and woman, they were incensed by it - but John and Paul hadn’t a care. They didn’t mean to be rude about it but basically it was tough shit.

it was tough too on Dot and Cyn, Dot simply had to accept the situation, but Cyn had a greater case of grievance. John was heading off without her when he could so easily gave waited for the art school holidays. (…).

That John was taking Paul, no one else, accentuates the renewed closeness since Stu quit The Beatles. They were the Beatles force, an unstoppable and authentically powerful pair. “Lennon had the attitude”, Wooler said, “and taking his lead from Lennon, McCartney could be similar. At times they reminded me of those well-to-do Chicago lads Leopold and Loeb, who killed someone because they felt superior to him. Lennon and McCartney were superior human beings”

"You’d always see them together, in the pub or walking along the street", says Johnny Gustafson of the Big Tree. "They were a duo, and seemed each other’s equal". Bernie Boyle, the young lad hanging around with them at every opportunity, says, "They were like brothers, with John as the elder and Paul’s mentor. They were so tight it was like there was a telepathy between them: on stage, they’d look at each other and know instinctively what the other was thinking"

They were brothers. They were the Nerk Twins, and now they were taking a break from The Beatles and gofin off to Spain. 

Gustafson happened to bump into them the day they left, Saturday, September 30. “They both had bowler hats on, with the usual leather jackets and jeans. They said they were off to Paris, so I walked down to Lime Street station and watched them go. They were an incredible pair: always great fun, irreverent and so close. - Mark Lewisohn, Tune In: The Beatles: All These Years (2013)

As was written in this post: That last picture is one Paul took of John sleeping in Paris. From what I remember of a performance he did of ‘Here Today’, and earlier comments, this picture hangs framed on a wall in Paul’s house.

Unconfirmed quote (may or may not be true): 

"He must have been fond of me to spend that money. He let me have all the banana milkshakes I wanted.”  - Paul McCartney

In January 1964, only a few scant weeks before the Beatles took America by storm, the band mates settled in for an extended stay in Paris. For the group, the Parisian visit proved to be a magical experience, with the Beatles playing 18 shows at the Olympia Theatre between Jan. 16 and Feb. 4 (source).

The Beatles were staying at the George V Hotel at the time. John and Paul composed "Can't Buy Me Love", "I Should Have Known Better" and "If I Fell" on the piano.

The photo Paul took of John (in the "Eyes Of The Storm" book):

1966: Paul, his girlfriend Maggie McGivern, John and Brian Epstein spend 5 days in Paris. "All of them flew into France separately — Lennon had been filming abroad and Epstein had been away on business. Maggie and Paul, she says, traveled apart ‘as part of keeping the relationship secret’. During the five-day trip the foursome stayed at the same Paris hotel where she and Paul shared a luxury suite. ‘It was a marvelous holiday,’ she says. ‘. . . just walking around the streets of Paris.‘My abiding memory is of me, John and Paul lying under the Eiffel Tower, gazing up at it. We couldn’t go up because we would have been recognised, and we were masters at the art of avoiding people." [x]

1969:

Hoping to get married in France, John Lennon and Yoko Ono flew to Paris on this day [16th March].

The couple had decided to marry on 14 March 1969, two days after the wedding of Paul McCartney to Linda Eastman; whether it was in response to this event on some level is open to conjecture.

On McCartney’s wedding day Lennon and Ono were travelling to Poole in Dorset, where he introduced her to his Aunt Mimi. During the journey he asked his chauffeur Les Anthony to go to Southampton to enquire about the possibility of the wedding being held at sea, on the cross-channel ferry to France.

(source)

“On March 12, Paul married Linda Eastman at Marylebone Register Office in London, amid scenes of hysterical grief from his female fans. None of the other Beatles was present. The news reached John as he and Yoko were driving down to visit Aunt Mimi in Poole. Yoko’s divorce decree had become final a few weeks earlier, and, in a resurgence of Beatle copycat, John told her they, too, must get married as soon as possible” - Philip Norman, John Lennon: The Life (2008)

"We chose Gibraltar because it is quiet, British and friendly. We tried everywhere else first. I set out to get married on the car ferry and we would have arrived in France married, but they wouldn’t do it. We were no more successful with cruise ships. We tried embassies, but three weeks’ residence in Germany or two weeks’ in France were required." - John Lennon

1974:

“After a late lunch, Linda launched into a long paean to the joys of living in England. When she was finished, she turned to John and said, “Don’t you miss England?”

“Frankly,” John replied, “I miss Paris.””

— May Pang, Loving John (1983)

1978:

Wings album "London Town" is released. It includes the song "Cafe on the Left Bank", the lyrics of which clearly refer to John and Paul's trip to Paris.

Late 1970s (maybe 1978?): John is singing to Paul about Paris in a home recording. Longer version

1970s: John writes "Skywriting by Word of Mouth", a book that would be released in 1986. One story is about sex he had with a woman in Paris. Here it is. As anon noticed here: "...the woman is called Amie L'Nitrate and Amyl Nitrate is a reference to poppers. He talks about grabbing her 'pomme de frites.' Her potatoes? He uses the term 'tread lightly on some loafers' which is an old euphenism for being gay. Amie says they should have sex to God Only Knows. Then John says their relationship ended in a seething rage but he still thinks of 'her.'" @sgtsaltsband concluded in the same post: "so he writes a story about PARIS ( where he and paul went on a trip for his 21st bday and never stopped talking about it ) , in the HOTEL where the Beatles stayed later on [Hotel V in 1964] , names the girl after POPPERS ( a drug commonly used by gay men during sex ) , the girl wants to have sex to PAULS fave song and he uses this PHRASE." Also: this is an excerpt of the story:

"Boogie" is a slang word for sex or dance (also, "Born to Boogie" is a 1972 movie starring Marc Bolan, Elton John and Ringo Starr). "Band on the Run" is a Paul McCartney and Wings' album which John loved. "Sue you sue me" can be a reference to to the Beatles' legal and business disputes and the fact that Paul sued John, George and Ringo in December 1970, and to "Sue Me, Sue You Blues", a song by George.

(thank you @menlove for uploading the story and pointing out interesting words!)

1994 - Paul inducting John to Rock and Roll Hall of Fame:

“And then on your 21st birthday you got £100 off one of your rich relatives up in Edinburgh, so we decided we’d go to Spain. So we hitch-hiked out of Liverpool. And we got as far as Paris, and decided to stop there for a week. And eventually got our haircut, by a fellow named Jürgen, and that ended up being the ‘Beatle haircut’.”

I also remember watching an interview with Paul about his album "Memory Almost Full" (2007). Thank you for adding, @ringompreg!

(it's like 7 minutes in) Q: There is a very beautiful song called "The End Of The End", the way you talk about your whole ending, and the lyric goes: "It's a start of a journey to a much better place." You mean, better than England? Paul: It's basically a start of a journey to France. Or Spain through France. Yeah, that's what it is. It's a much better place, Paris.

2023 - Interview for Today - Interview with BBC Radio 4's Today programme

Q: Well, let’s see how the good looking boys took Paris by storm as we come into this next room, and the show in Paris was just two years after you and John had gone there hitchhiking. Those two trips must have been incredibly different. Paul: Oh, yeah. I mean, it was just the two of us. And we didn’t know Paris. So we just wandered around, and went to various things, went to the cinema. We just did a lot of stuff together. You know, we went to the cinema, it was an old film called The Time Machine, the original one. And what was amazing for us, the kind of thing that builds a friendship, we were up in the balcony upstairs. And two French guys came in and sat right in front of us. And we thought they got out some gum to share. But it was a piece of garlic. I mean, oh, God, the smell. And we feel like Liverpool people is like, no, that’s not good. And they just share in a little clove of garlic each you know. So you know, all those kind of memories come back when I see these pictures.

Also worth mentoning:

"All You Need Is Love" begins with La Marseillaise.

"Picasso's Last Words (Drink To Me)" contains French-language speech by BBC broadcaster Pierre Le Sève.

Just when I think I’m winning When I’ve broken every door The ghosts of my life Blow wilder than before