Hi hi! So I participated last year but tapas prom is something that was started by Mochi the artist of Fresh off the Press the first one was maybe one or two years ago but there's no set date for it each year and usually only comes together if she's up for it (being the organizer and all)
I remember either earlier this year or maybe some time last year she'd brought up some theme ideas and potential dates but I don't really know if any kind of final date was set. It's a fairly casual event and really started among a group of artists whom I had the chance to interact with. I'm not sure if there's gonna be one this year but feel free to follow her if your hoping to catch any info about it plus definitely check out her art
But as far as how it goes is that a theme is chosen, last years was spring/floral, and then everyone who wanted to participate was invited to make an comic episode/illustration with there mc's dressed to fit the theme. Some folks did comic prom-posals (leading to some cute and funny crossovers) as well as other prom activity themed artworks. Afterwords they were all shared to the organizer (Mochi) and then those that were most liked might have gotten a cameo artwork in a sort of cumulative piece.