Your first payment is due the first day of the month after your loan closes. For example, if your loan closes September 15, your first payment is due November 1. If your loan closes April 28, your first payment is due June 1.
Keep in mind thatl although your payment is due on the first day of the month, it's not considered late until after the 15th day of the month. I often recommend that you set up your mortgage payment with your online banking to be paid by the 12th or 13th of each month. Make sure to check each month that the payment has actually been made by your bank.
When you close escrow, you will be paying prepaid interest from funding date to the end of that month. Example: if you close escrow on September 15, you will pay prepaid interest from September 14 to September 30. Your first mortgage payment due November 1 covers interest from October 1 to October 31.