The WEEKNUM function is a date and time function which takes "serial_number and return_type" as argument and returns the number of a week, which denotes where the week falls numerically within a year. The weeknum function is mainly used to get the week number from the given date.
=Weeknum (serial_number, return_type) In the above syntax, the argument Serial_number is a number which represents the date within the week. The dates should be given in the format of Day/Month/Year i.e. 7/8/2006 which denotes 7th August of 2006. Return_type is a number that denotes the day in which the week begins and it is optional. If the return type is given as 1, it means that the week begins on Sunday and the weekdays are being numbered from 1 through 7. If the return type is given as 2 it means that the week begins on Monday and the weekdays are also numbered from 1 through 7.
Let us learn how the weeknum function works from the below video.