The day before a Sketch Fest, I recommend the following:
Check out the available work! All of those sketches will be removed from sale tomorrow before Sketch Fest begins. Some of them may be re-listed, but this is your last chance for many of them!
Jot down a few prompt ideas. Once Sketch Fest really starts, there's often a moment of 'dang, I had this GREAT prompt idea...' and then you spend a bunch of time trying to remember it instead of prompting or sketching. :P
Collect some references. Look for generic things you might be able to use for a variety of different topics.
Set up a work area. Clean off your desk, collect your art supplies and paper, or if you're working digitally, make a new folder for this Sketch Fest and think about running your virus scanner or even de-fragment your hard drive the night before. Make a playlist, or put out some discs that tend to jumpstart your creativity. It's more fun to work in a space that isn't cluttered with distractions.
Let people know! Make sure your family knows you want a few hours (or even all 24 of them!) for this event. Ask for rainchecks on social things, and maybe even ask them to play along and leave prompts! They may end up enjoying Sketch Fest as much as you do!
Go comment on some of the finished work. These pieces often get many fewer comments than the sketches posted in the heat of a fest, and some of them are simply AMAZING. Go check them out, and give them some comment-love! (And look! You can REPLY to comments now, too! And replies to new comments [though not old ones, sorry] will send an email to the person who left it, so you know they will get your thanks for their comments or answers to their questions, etc.)
Look back over your previous Sketch Fest offerings and analyze them. Which ones do you like the best? How come? Which ones were the most fun? Do you see any areas where you'd like to see more improvement? Keep these things in mind as you tackle your next batch of sketches.
If you'd like to buy artwork, make sure to add credits to your account! You have to be logged in to do this, but it means you get first crack at all the artwork available!