Croissant is a monthly recurring membership, so it will automatically renew on the same day each month unless it is paused or cancelled. For example, if you sign up on the 15th, your membership will renew on the 15th of each month after that.
There is no long-term commitment for Croissant. You are welcome to cancel your membership or put it on hold at any time. You can do either of these things from your Account Settings.