Technically you can start when you want, you can plan everything out this weekend and start monday, or whatever you want. This weekend is for those few who are wavering or who are on the fence about starting, and it gives the judges some breathing room. as it stands everything is pretty much finalized.
also you have until the deadline to change whether you're entering competitive or just for the gallery, if you have some sort of situation that you aren't sure how things will go this week or something, then just see how it goes, but if you're simply unsure about whether or not to enter, personally I'd say go for it, it's fun and a great way to refine your skills as a Game Dev. The judging will be based around skills that any dev should have and overall it's a lot of fun, and you never know how good things might turn out.