Memberships automatically recharge each month on the same calendar day (anniversary date) you signed up on. For example: if you signed up for your membership on November 5th, your next bill date would be December 5th and then January 5th.
There can be some exceptions, such as signing up at the very end of the month on the 29th, 30th, and 31st. For example, if you sign up on Jan 30th, your next bill date will be Feb 28th. Billing would then continue on the 28th moving forward.