Hello everyone!
First off, I want to say I'm excited to see what you make! Last year had some amazing entries, and I already know this year is going to be even better!
I wanted to make sure to clarify something with the jam!
As stated in the rules on the front page of the jam, you're allowed to use premade assets/asset packs for your game. This would be any asset pack you have license to use for non-commercial or commercial purposes- as long as you're allowed to use what you're using, you can use them. (So not sprite rips from existing games, etc)
This brings up the question- "Am I allowed to start working on sprites/sounds/music early?", and I think the only fair answer to that is yes- because you can use premade asset packs, it's also okay to start working on assets before the jam. You can write design documents, draw out ideas, make assets, etc. The only thing you cannot do is start coding your project/make a project file.
As long as you start coding your project when the jam starts and stop coding your project when the jam ends, you're a-ok!
I hope that clarifies things a bit! I really want people to be able to make the best possible game they can within the duration of the jam.