To remain eligible for all awards, substantially all of your project needs to be created at CodeDay. For example, the following would not be allowed:
- Working on an app you started in school.
- Continuing to work on a game from last CodeDay.
- Modifying assets in an open-source game.
- Using a large amount of art you created earlier.
On the other hand, these are definitely ok:
- Using an open-source framework or library, provided it's free and wasn't created specifically for your app.
- Using one or two sprites you'd created for a past game.
- Copying a couple of functions from a previous app you worked on.
- Concept art, notes, storyboards, and notes.
Exactly what is an isn't allowed is defined in the CodeDay rules, but judges are ultimately responsible for assigning awards, and they have a lot of leeway in interpreting the rules.
If you don't care about awards...
Awards aren't super important at CodeDay (and there are no big prizes), so if you don't care about winning the "Best in Show" or "Best in Class" awards, you can do anything you want before CodeDay! Just be upfront and honest about what you created in advance.
You'll still be eligible for any special awards the judges create.