When is the monthly payment considered late?

Generally when you loan is due on the 1st, you have until the 15th before you incur a late charge. Once the loan is 30 days past due, your payment will be reported to the credit bureau as 30 days late. So if you made your payment within the month due, and it posted, you are only liable for the late payment and will not affect your credit or housing rating.

I support and agree with Chris Gummerson's answer. The only think I would add is if anyone does a direct verification with your mortgage company they may report Late Payment Fees you have paid. In todays times and underwritings you may be asked about the cause of those late payments.