If you upgrade from a free account, you should be billed when you complete the upgrade process. Then you'll normally be billed at the same time from month to month unless you cancel your premium service. After that, if you ever renew again, the billing date will change to the new date you sign up.
Alternatively, if you start a 30 day trial, you'll be required to submit your payment information and will be billed at the end of the 30 days if you don't cancel the trial.
More information on a lot of this is in the "payments" section of the FAQ here.
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