I always thought you had to say "Rabbit Rabbit" the last thing before going to sleep on the last day of the month, and the first thing upon waking up on the first of the month. The former is easy, because you can just say it again if you have to say something else before you fall asleep. The latter is harder because it's hard to remember when you first wake up.

Now I just say it when I think of it around the first of each month.