ISOWEEKNUMReturns the number of the ISO (International Organization for Standardization) week of the year where the provided date falls.
ISOWEEKNUM(DATE(1969, 7, 20))
date – The date for which to determine the ISO week number. Must refer to a cell containing a date, number, or function returning a date type.
When entering the date, use the DATE function, as text values might return errors.
Per the European system for numbering weeks (also known as ISO 8601):
Weeks begin on Monday and end on Sunday.
Week 1 of the year is the week containing the first Thursday of the year.
As a result, if the first Thursday is after January 4, at least one of the following days, January 1-3, will have an ISOWEEKNUM of 52 or 53.
Similarly, if the first Thursday is before January 4, at least one of the following days, December 29-31, will have an ISOWEEKNUM of 1.
If the first Thursday is January 4, December 29-31 of the previous year will have ISOWEEKNUM of 52 and January 1-7 of the current year will have ISOWEEKNUM of 1.
WEEKNUM: Returns a number representing the week of the year where the provided date falls.
WEEKDAY: Returns a number representing the day of the week of the date provided.
DAYS360: Returns the difference between two days based on the 360-day year used in some financial interest calculations.
DATE: Converts a year, month, and day into a date.
DATEVALUE: Converts a provided date string in a known format to a date value.