You are billed in advance based on the length of your subscription. You will always be billed on the same day of the month. This gets slightly complicated for months of unequal lengths. What happens in those cases is we push back the renewal date as necessary, which is best shown by an example.
Say you buy a monthly subscription on December 31st. Well, January still has 31 days, so the next renewal billing happens on January 31st. But February only has 28 days, so the next renewal billing happens on the 28th. And, from there on out, it remains on the 28th of all upcoming months. The same idea holds true for both three and six month subscriptions.
Renewal billing will happen automatically but you will receive an email before the renewal is charged to the same payment method you purchased with.