Is it necessary to claim a prompt by June 1st in order to participate, or might someone be able to make a later prompt selection and write something for it? *looks hopeful*

I ask because I've love to participate, but I have 2 stories due in June and so I'm reluctant to commit to anything else yet. I fear that if I choose a prompt before finishing those projects then the idea might run away with me and I'll end up writing 20,000 words about the librarian (or something) instead of the help_brazil fic I've promised.