^^ Splatfests are like a weekend-long event where you get presented with a one-time choice between two categories (e.g. the weekend just gone was "Sleeping" vs. "Eating") you make your choice and become part of that team for the weekend (it cannot be changed)
During the event any lobby matches you play in will place you specifically with other members of the team you are on (and against a team comprised of entirely members of the other team). The winning team of each match score 1 point for their Splatfest team.
At the end of the weekend the final scores are calculated based on the sum of the percentage of "votes" for each team, plus 2x the percentage of wins the teams had over the weekend. (So for e.g this weekend team "Eating" had something like 69% of the votes, and team "Sleeping" had about 55% of the wins, but overall this meant that the "Eating" team won the Splatfest (as 69.0 + 45.0*2 > 31 + 55.0*2)
TL;DR It's a little gimmick where you pair off into 2 big teams for the weekend
I'm still only lvl 6... enjoying the game though it can be a bit rage-inducing at times; some weapon combinations seem to be quite difficult to deal-with (but I'm sure it'll all come with experience). The matchmaking during Splatfest seems to struggle a bit more than usual so it wasn't uncommon to end up having a full-team of lvl-20 guys going against a team of sub lvl-10s and it being a complete stomping...
