You get billed each month or year (depending on what plan you selected) on your billing date. The credit card on file for your account will be charged for your upcoming billing period.
Your billing date is the date your bill is generated. This is typically the date when your 30-day free trial ended. If your billing date is on the 31st of the month and there is no 31st that month, you will be billed on the 30th of that month (or the 28th/29th if it is February).