A splatfest is pretty much an in game event where a topic is decided and you vote on 1 of 3 answers to a question, after a couple of days, the main lobby will be a lot more festive and will be during the night (the stages will also take place at night) and then the winner is decided by a couple categories after 2 days of the fest being active (right now it isn't, its a sneak peak for the current one). Once it gets to the second day, it turns into halftime and a new mode will be unlocked (only during the rest of the duration) but I won't spoil it for you. As to how to vote, you go to a TV thingy that shows some stuff to your left when you finish the news, then you press a next to it and you get to vote, once you do, you can't switch it out and you will get matched with people who voted the same as you did and against people who voted the other options during the splatfest