Confirm your due date and time. Some lenders require payment to me made before the close of business on your due date. This means that if your payment is due on March 1, the lender will require the payment be received and processed before the close of business on March 1. Other lenders require payments to be postmarked but not necessarily posted by the due date. Find out the specific rules by reading your loan document or asking your lender.
Count the days from the due date. If your payment is due on March 1 and it is March 3, then your payment is two days past due since you count two days from the first to the third. Weekends and holidays count as days, so those factor in to the calculations.
Understand the implications. A payment is reported as late on your credit report once you are 30 days past due. There is a separate category on the credit report for loans that are 60 days past due and 90 days past due. The later a payment is, the more adverse the impact on your credit score. Furthermore, when your account reaches 90 days past due, many lenders will begin collection proceedings on the account.