Grace period
The grace period is the window between your statement closing date and your payment due date, if your card has one — card companies don’t have to offer it, but they must have procedures designed to ensure your bill is mailed or delivered at least 21 days before the due date (Regulation Z). If you pay your statement balance in full by the due date, you usually owe no interest on purchases.
Why it matters
It matters because it lets you pay for purchases without interest each cycle, but the protection only holds when you pay the full statement balance; carry even a small amount and you can lose the grace period on new purchases.
Example
If your statement closes on the 1st with a $400 balance and you pay all $400 by the due date on the 23rd, you pay zero interest on those purchases.