Week and Day number

The Excel WEEKNUM function takes a date and returns a week number (1-52) that corresponds to the week of year. The WEEKNUM function starts counting with the week that contains January 1. By default, weeks begin on Sunday. .

 

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The WEEKNUM function starts counting with the week that contains January 1. The return_type argument controls which day of the week is used to begin a new week number. Return_type is optional and defaults to 1.

With a return_type of 1-17, week number 1 in a given year is assigned to the week that contains January 1. With return_type 21, week 1 is the week containing the first Thursday of the year.

Return Type /  Week Begins
1 (Default) – Sunday
2 – Monday
11 – Monday
12 – Tuesday
13 – Wednesday
14 – Thursday
15 – Friday
16 – Saturday
17 – Sunday
21 – Monday (See link below)

https://en.wikipedia.org/wiki/ISO_8601#Week_dates

Syntax 
=WEEKNUM (serial_num, [return_type])
Arguments 
  • serial_num – A valid Excel date in serial number format.
  • return_type – [optional] The day the week begins. Default is 1.