The gap between the statement cut-off date and the payment due date during which no interest accrues, as long as you pay the full balance. On most Mexican credit cards it runs 45 to 55 days. If you only make the minimum payment, the benefit disappears: interest is charged from the day of the purchase.
Your card gives you a grace period — usually 20 to 50 days — between the purchase and the payment due date. If you clear the full balance before the deadline, you pay zero interest on those purchases. That's the smart use of a card: free short-term financing.
The trick almost nobody explains: interest-free days only apply if you carry no revolving balance. The moment you leave a balance unpaid, you lose the grace period and new purchases start accruing interest from the day you make them. That's why «paying the minimum» breaks the whole benefit.