Each circuit is meant to be repeated 3 times. For example, May 4th, do 10 Crunches, 10 reverse crunches and a 20 second plank, no rest in between moves. Then rest for 20 seconds and repeat the round, then, rest for 20 seconds and repeat the round for a total of 3 rounds.
This calendar is from my May 2015 challenge, but feel free to use it for any month, just follow it like a 28 day challenge. Enjoy!