Hello, so we are supposed to start only on August 1st? Just asking cause it's like we could start now since there is no theme.
You can always technically start a game jam if there's no theme being held back... but it's against the spirit of the thing, and not how it's intended to be done. Kind of takes the fun out of it.
What you can and probably should do -- at least if I'm understanding right, and this is the same for a lot of other jams -- is start your planning now. I'm in the middle of defining my game loop, gathering assets, and laying out how I'll organize my UI and transitions. There are also likely to be details to work out specific to your development environment, depending on what you'll be using to write your game, and those are problems you can research without actually starting your project.
No code gets written that'll actually go into the game, but prep is important, particularly when you only get two weeks of actual implementation time.
As internationalfish says, there's no harm in prepping yourself and your tools for the jam. If you consider that a lot of people won't have to. For example; people using Unity, GameMaker, etc have very little prep as all the tools are integrated, compared to those not using IDEs who have to sort out all sorts of dependencies, libraries, build systems, etc. Things like that shouldn't have to be done to any specific jam time, if they was you'd pretty much end up doing the same thing again for every jam.
As for starting early, you could do that even for a themed jam. But at the end of the day, there's no prize, only the fun and challenge you have while doing it and the sense of achievement at the end. Yes there's some friendly competition, but there's nothing to "win", so if you "cheat", well that's just on your conscience because no one else is going to know and/or care accept you. :)
For example; people using Unity, GameMaker, etc have very little prep as all the tools are integrated, compared to those not using IDEs who have to sort out all sorts of dependencies, libraries, build systems, etc. Things like that shouldn't have to be done to any specific jam time, if they was you'd pretty much end up doing the same thing again for every jam.
If you're doing the same jams all the time, yeah. I've personally never made a 64x64px game, and Unity doesn't exactly handle that perfectly out of the box, so that's something I've been looking into beforehand.