In MS-Excel, Day function returns the day of a given date, which is represented by a serial number. The return value is an integer ranging from 1 to 31. Syntax DAY(serial_number) Serial number is the date of the day you are trying to find. Dates should be entered by using the DATE function, or as results of other formulas or functions. Example: Assume the cell A2 consists of the Date 10-Apr-09, then DAY(a2) returns the value 10.