When you purchase a new subscription, your credit card will be charged immediately for the first month (or quarter or year depending on selection) when you check out.
Your subscription will renew and your credit card will be charged on the same day of the month that you started your subscription. For example, if you purchased a monthly subscription on May 6th, your renewal will be charged on June 6th.
For subscription purchases near the end of the month (29th, 30th, 31st), renewals and charges will happen on the last day of the month, or your renewal day, whichever comes first.