To a certain extent, you are allowed to prepare for the game jam before it officially begins. This includes what can be referred to as pre-production, which involves brainstorming ideas, storyboarding, and creating a game design document. However, you are not allowed to begin actual production of any assets or start coding until the game jam officially begins.
It's important to note that even creating art sketches in advance could be considered cheating and is therefore prohibited. The purpose of a game jam is to challenge participants to create something new and innovative within a set time frame, and pre-production is allowed only to a certain extent to help participants plan and organize their ideas, not to give them an unfair advantage over other participants. So while you can start planning and ideating before the game jam starts, you cannot start creating any tangible assets or start coding until the official start time.