Yes. Prior to May 19th, you can do any pre-preparation you might need such as looking up for potential references, thinking about possible setup/design, etc. But you got to remember that the theme and, sometimes, the bonus theme/rules (gives bonus points) can go completely against your ideas, hence that's why you shouldn't get stuck with 1 great idea, but just some general brainstorming.
Think of it as the time used to practice by making a bunch of cakes or looking up recipes and techniques around prior to a cake contest. Knowing which tools you're need during the contest is kinda essential unless, like me, you go in blind (for an additive personal challenge).
A good list of example of stuff you can prepare ahead in this kind of Jam:
• A list of Public Domain (CC0) musics files OR musics you have, yourself, made. (No remixes or any derivative works that is too close to any existing copyrighted musics.)
• Pill up a bunch of animations files that could be useful. For example, you can download a bunch of animation from mixamo.
• Preset of stuff you have already build in concept such as codes or systems.
While some big Jam might refuse that you bring in your own pre-made codes, this kind of Jam allows you. It even gives you an example of what you can bring in with the package it gives for free that includes some basics for some games. If you got some codes that works for something you want to make during the Jam, nothing is stopping you from copy-pasting the code right away.
On May 19th, the theme and bonus theme/rule will be given.
From that point, you start working on the actual implementation of the game itself so that you can have a working client by the 29th.
Before the Jam close on the 29th, you'll have to publish a working client (.exe file) into itch.io. This allows you to submit the project to any active Jam you have joined. (It's all in the itch.io publish menu.) This Jam also requires that you share the build (project files) which it's highly recommended that you separate the 2 set of files in different .ZIP containers so that people can download just the game if they want.
This is also why you can't use copyrighted stuff in the JAM (unless you own the license for redistribution which, 99.9999% of the time means that you made it yourself.) Even free stuff online can be restricted on the redistribution of the work hence you got to look up for stuff specified with the CC0 mark (meaning it's on the public domain and free to use, re-use and redistribute or use in commercial works).
If you're uncertain if you can use certain assets in this JAM, just explain and ask the person who made the asset if it's alright to put the file in your project that will be distributed for free. If you got a positive answer, it's all OK. For example, someone might agree to the redistribution if credits are given. As such, you can include a PDF or .txt files containing the credits mentioning the requirement for the crediting in using the asset. (This is often the case on free content offered on the web. It's often called CC with Attribution. CC means Creative Common)
This is also why it can be a good thing to start looking and loading up a bunch of musics file ahead, unless you truly plan on making it yourself during the JAM or unless you already got a good source of CC0 or CC with Attribution musics on hand. ;)
This is the example I have made for the GameDev.tv Game Jam 2022 last year: