Credit card answers from the Federal Reserve bank
It might differ from card to card, but it's the Grace Period that leaves you interest free.
The FRB had a pretty good defiition of the Grace period
Quote:
The grace period is the number of days between the statement date and the due date during which you can pay your bill without incurring a finance charge
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Basically, if you want to but the laptop interest free - buy it on your statement closing date, and you've got 27 days or so to pay it off.