Typically, it is on the 1st day of the month 30 or more days after closing. For example, if you were to close this week, within the month of September, your first payment would be November 1st. If you were to close next week, your first payment would be December 1st.
Any length of time from closing to the first payment date longer than a calendar month will be collected as interim interest at closing. This should have been disclosed to you, based off the projected closing date (aka "drop dead" date) on your purchase agreement, as it effects how much you might have to bring into closing.
Do not miss your first payment!! That results in automatic default and foreclosure.