Per diem is Latin for "for a day." So naturally, if you add the word “interest”, per diem interest means the amount of interest for one day.
Most mortgage lenders will charge you interest on a loan from the date of the closing (settlement date) to the end of the month. For example, if you close on the first day of the month, you will pay interest on the loan from the 1st until the last day of the month. Count the closing day.
If you settle your loan on the 15th day of the month, and the month has 30 days, you will pay 15 days' interest on the loan.
And one more illustration, if you close on the last day of the month, you will pay interest for one day, the day of closing.
In most parts of the country, you pay interest on the day of closing. Learn more about Per Diem Interest