The WEEKNUM function converts a serial number to a number representing where the week falls numerically with a year. For example, the week containing January 1 is the first week of the year, and is numbered week 1.

WEEKNUM takes 1 required argument and 1 optional argument:

Syntax: WEEKNUM(serial_number, [return_type])

#1)
Using the WEEKNUM function:
#2)
The arguments for the WEEKNUM function are:
Argument Required? Description
serial_number Required A date within the year. Dates should be entered by using the DATE function, or as results of other formulas or functions. For example, use WEEKNUM(2014,3,17) for the 17th day of March, 2014. Problems can occur if dates are entered as text.
return_type Optional A number that determines on which day the week begins. The default is 1.
System 1 The week containing January 1 is the first week of the year, and is numbered week 1.
System 2 The week containing the first Thursday of the year is the first week of the year, and is numbered as week 1. This system is the methodology specified in ISO 8601, which is commonly known as the European week numbering system.
Possiblereturn_typeargument values:
#3)
A couple more things:
If serial_number is out of range for the current date base value, a #NUM! error is returned.
If return_type is out of the range specified in the table above, a #NUM! error is returned.

Summary

The WEEKNUM function converts a serial number to a number representing where the week falls numerically with a year.
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