You will be charged at the beginning of each billing cycle:
Annual subscriptions: You will be billed once a year, the first payment will be done immediately after upgrading to a paid plan (this is your subscription anniversary date). You will be billed every year on your anniversary date.
Monthly subscriptions: Your credit card will be charged at the beginning of each month. For the first month you will pay immediately after upgrading to a paid subscription, you will pay a pro-rated amount for the remaining days in the month.