A grace period is NOT an extension of your payment due date. If you are not carrying a balance, it is the period you have to pay new balances before interest charges apply.
If your payment is received past the due date, it may be considered late. It's better to mail your payment early so it gets in before the due date. Better yet, you can often arrange to have payments made electronically from your checking account on a set day each month. It's a great way to be sure your bill is paid on time. Enroll in Citi's AutoPay service to automatically pay your Citi credit card bill from a linked bank account.
If you don't pay your balance off every month, develop a regular payment pattern. Information about how you pay your bill is regularly sent to the credit bureaus. If you don't send in at least the minimum payment due each month, a negative mark could appear on your credit report, even if you pay the balance in full the next month. The first thing companies look for when they see your credit report is whether you pay your bills on time.