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Well, we've already seen parts of it, scattered throughout Episodes 1-8. Just with changes to the order, characters or context.

MARCH-MAY

Season 4 ending. March 29th, 1986.

Remember how El wasn’t talking to Mike at all at the end of Season 4? And how for most of Season 5, they weren't together? Mike and El break up. {Vecna later uses this memory to tell the story of Jonathan and Nancy's breakup.}

Eleven: "I hate [Eggos]"; "You are always exactly six minutes late." "You wanted honesty." Friends don't lie.

Mike: "That spring break, when you were in Lenora..." "[physics mention]"; "I just felt like I was doing something wrong. Like I was going against the natural order of things."

Mike: "It made me feel safe, but also... I dunno." Eleven: "It was suffocating." Mike: "Yeah."

Will + Robin start hanging out. {This probably happened the same way – after he saw Rovickie kiss.}

The Byers are staying with the Wheelers. Mike and Will share the basement, which explains why they forgot to set an alarm in The Crawl. Mike is ALWAYS late!!! Every season except 5!!!

Will: "It's always [Mike's] fault!"

JUNE-JULY

Robin and Will have their coming-out talk from Shock Jock! They're quickly becoming friends, supporting each other. They both agree to come out to someone new. Will decides to start small, and Robin wants to go big.

Will: "You wouldn't happen to have any more of that truth serum left, would you?"

Will quietly comes out to Mike, Jon and Joyce – his own choice. This explains why the Duffers said they originally wrote the scene this way, but changed it. Right after Will's coming-out, in a private moment, Mike gives his apology from the tower in The Rightside Up.

Mike: "Hey, um… I’m sorry. I mean, not sorry about what you said. That came out wrong. I mean, not ‘came out’ wrong! Jesus Christ!" LMFAOOOO

Because of that coming-out heart-to-heart, Mike and Will become much closer again.

Robin comes out to everyone, so she can “double her audience”. That's why there was so much rambling in that scene, and why Tammy was mentioned! Robin says she likes girls, talks about her "Tammy", and maybe even mentions finding her "Vickie".

Mike has an epiphany. Perhaps he had his Vickie all along. 

AUGUST-OCTOBER

Will talks to Robin. He asks how she knew that Vickie wanted to date – their talk from the hospital. He immediately starts flashing back to all their little snowballs... "a brush of the knee, a bump of the elbow, a shared look."

Mike talks to Robin. Maya Hawke said her character would "mentor" TWO others this season. What if Mike's the second? He's amazed after her coming-out, and asks her for a bit of advice. Probably also about Vickie, haha.

Mike gets Jonathan's help to make something special: a mixtape for Will. Jonathan makes it, and gives it to Mike. {Later, Vecna uses this memory to create the scene where Murray gives Jonathan the mixtape for Nancy.} Also – Charlie Heaton said Jonathan would help his brother "get something he wants" in Season 5. This concludes Jonathan's arc of noticing what's going on with Will and Mike, but never intervening.

In the Wheelers' basement, Mike gives Will the mixtape. This is the avalanche; the thing that made it obvious.

Mike: "One last confession. A big one." He pulls out the mixtape. "I figure I've screwed up enough with you. It'd be nice to get something right for once in the end. Here." {Later, Vecna uses this scene for Jonathan and Nancy's breakup, where he takes the ring out and throws it away, in a confusing un-proposal}

This could explain the cutgate claims that Mike and Will were apparently supposed to have a basement scene together, and that music was supposed to play a fundamental role in scenes – e.g. "Just Like Heaven" by the Cure.

Possibly birthdaygate too; a late present?

I'm just speculating here, but... I think they'll use this to recontextualise the Purple Rain scene. Purple Rain plays from the mixtape as Will says Eleven's words from Episode 8. Quiet and intimate, not life-or-death.

Will: You understand me better than anyone. You always have. From the day we met, you've seen me. The real me. {Later, Vecna uses these words to evoke Mike's guilt – about Eleven's fake death, and his unreciprocated romantic feelings. This is to keep him trapped in the Rightside Up, AHHHH–}

After this scene, Mike and Will are either together, or plan to be together. This does multiple things narratively. It gives Vecna something he can use to manipulate them, it will also be one of the memories that helps Mike escape the illusion, and if Mike and Will were together throughout Episodes 1-8 but just didn't remember, those episodes are totally recontextualised!!!

ALSO!!!! This explains why the Duffers said that Eternal Sunshine Of The Spotless Mind was a big inspiration for Season 5. Two lovers' memories of each other are erased, but they find each other in the end, with the help of a CASSETTE TAPE!!!!!!!!

Apologies for the avalanche of exclamation marks.

NOVEMBER 6TH, 1986

As @mixu suggested, Mike and Will may have planned to go on a date on or after November 6th. This is supported by:

The video clip of Mike saying "You know I had dinner plans, right?" Dustin replying, "With who?" and Mike avoiding the answer. [Unless Mike and El have broken up, it should be obvious who...]

Not just that, but he's being forced to miss his dinner plans so he can rewatch edited clips of the show... i.e. edited clips of his own memories.

Just like Jopper, Lumax and Rovickie's date plans, Byler missed theirs because of Upside Down shit too.

Will attempts to save Mike using his powers during the battle with Vecna. [Possibly different ones than in Sorcerer.] But unlike in Sorcerer, he fails. {Vecna later uses this scene to give Will a winning moment [siphoning his own powers, hmm...], to sell the narrative that they were able to beat him easily later on}

Will: "[Mike]... Vecna, he's got [Mike]... we failed. I failed."

This explains why both Mike and Holly have the same head wound, and why Mike heard the clock chime and had multiple reaction shots to seeing Vecna – but nothing came out of it.

Will has his talk with Joyce from The Bridge. He's feeling so, so hopeless. Not only did Vecna get exactly what he needs to enact his plan, but... things were finally getting better between him and Mike, and now he's gone.

Will: "...Today is the day. The day Vecna's gonna act. I thought I had him, but the truth... the truth is I never stood a chance."

It's not over, though!

After that are Mike's altered memories, everything from Episode 1-8. Season 5 begins.

And after that... the final episodes!

(And hopefully, a somewhat happy ending :D)

Robin: It's a mandala! [...] Is it a mandala? Mandahla? Or a man-dala? Or a mendela? Or...