Kid iwaoi going trick-or-treating in pair costumes.
(Movie themes are their favorite. Along Godzilla and Star Wars, they also do Disney!)
Iwaizumi starts carrying candy or chocolate bars with him on Halloween ever since Oikawa trick-or-treated him when they were thirteen and Iwa didn’t have any sweet with him, ending up Oikawa playing pranks on Iwa the whole damn day.
Iwaizumi surprisingly trick-or-treats Oikawa in their first year in college and Oikawa of course has no candy with him. Oikawa stays on edge all day wondering what Iwa would trick him. But Iwa only asking for sweet kisses (and, ever more, candy-sharing kisses) when they are finally alone in their shared dorm room.
Halloween officially becomes Oikawa favorite holiday right after Christmas.