Maybe not at first, but as they become better friends, Jagna would invite Jonathan over to celebrate together St. Andrew's Day or "Andrzejki" as she calls it.
Ofcourse she doesn't believe the whole "magic" of the day. But it's tradition that's she fondly rembers, celebrating with some of her family. She thought it would be great fun to just sit together, eat some snacks and do few traditional fortunes about they future, ofcourse the most important and fun one being pouring hot wax from a candle through the hole in a key into cold water.(Seen in the picture)
I imigine that Jonathan at first only agreed to come and celebrate, since it's something his Great grandmother wouldn't approve of. Anything to spite that old witch and doing "magical" future telling definitely would make her spin in her grave. But as he sits there, eating snacks and cookies and doing the first fortunes, he finds himself enjoying it. Here was a girl who he slowly starts to accept as part of his life and even consider a "friend" doing something fun together. He let's some of his guard down and allows himself to just be silly. They have blast after that, specially as he cames with more nonsensical theories about what his wax represents.
But I think what hits him the most, it's fact she invited him. Here she was, sharing a tradition from her country with him. He feels... good? Happy? Jonathan was invited to share something special, for first time in his live. Well... He wouldn't mind being invited next year too.
After few years it becomes a tradition in they group of friends to celebrate together. Even having little contest of who can came up with more nosensical theory about what they wax represents. Winner gets a last cookie.