It works like this. Every day in April you write a new post, except for Sunday. Each day is a letter of the week. So the 1st would be a post with a letter A theme. The 2nd will be a B post theme and so on. I highly recommend trying it out. And you get to visit some really interesting bloggers that are also doing the challenge.
There is still time to sign up. Come on. Challenge your self. I'm number 589 in the list.