Membership renewals are paid on the First day of the month, either monthly or quarterly – this is a choice you can make when joining.
If you join part way through the month, your first month will be charged a prorated amount from the day you join till the end of that month. After this time, you will be charged the normal amount on the First day of the next month.
You can change from Monthly to Quarterly, or vice versa from you account dashboard.