I'm...shocked. This is the only place I know where Splatoon isn't that popular, unlike in GameFaqs.
Anyway, next week, it will be SplatFest in NA (Dogs versus Cats), then two weeks later for Europe (Pop Music versus Rock Music). Japan already had its SplatFest, though, but someone pointed out what you can expect from it:
*Best change is the base. Miiverse posts are everywhere. The stages are now set at night, and there's only one piece of music (including last 1 minute remix).
*You have to wear the T-shirt and can't even change clothing (just the body) during the Fes.
*Normal Turf War and Ranked Matches aren't available; only Fes Matches.
*Played like Turf War from a pool of 3 possible stages that haven't changed yet (we're over halfway through the event now, so unlikely to ever change.) This means your Rating for the Splatfest will be maintained for the entire event.
*I filled in the slots on my T-shirt prior to the event, but at some point during the week they got blanked out again (maybe after the new stage dropped?) Not sure it is worth filling them in beforehand.
*Fes Rank is handled by filling a new gauge. You get 1 Pt for covering 200pts of Turf, another 1 Pt for 400 (so win or lose, you should always score 2 Pts per Match) and then 3 Pts for winning. So the max you can score per match is 5. Reach Rank requires the following scores to Rank up - I won't bother with the name because they are likely to be localized anyway.
Rank 1 - 10Pts
Rank 2 - 25Pts
Rank 3 - 50Pts
Rank 4 - 99Pts
After that the Gauge just changes to MAX. So as you can see, even if you lose every single match you "only" need to play 92 matches, so long as you always score at least 400, in order to MAX the rank. There's no need to be sitting playing all day, then, unless you really want to feel like you are making a contribution to your team overall - something that doesn't appear to be displayed in the game at all.
*Seeing as you are matched in teams, if you choose "Continue" after a match you can in theory just stay with the same 4 players. You can't jump to Friend's games though so no real way to get matched with a friend. Matchmaking will still happily put 4 20s against a few 3s and 10s, so it's as an uneven experience as Turf War has become in general.