Date.WeekOfMonth Function in Power Query

The Date.WeekOfMonth function in Power Query (M language) returns the week number of the month for a given date value. The first week starts at 1, and the start of the week depends on the optional firstDayOfWeek parameter (default is Sunday).

Syntax

Date.WeekOfMonth(
    dateTime as any,
    optional firstDayOfWeek as nullable number
) as nullable number

The function has the following parameters:

• date: A value of type date from which you want to find the week number within its month.
• optional firstDayOfWeek: A number from Day.Monday (0) to Day.Sunday (6) specifying the first day of the week. If omitted, the default is Day.Sunday (6).

Return Value

• Returns a number indicating the week of the month starting from 1.
• Returns null if the input is null.

Key Points

• Week counting starts from the first occurrence of firstDayOfWeek in the month.
• If the date is in the first partial week of the month, it's considered week 1.

Example: Default (week starts Sunday).

Power Query M

let
    Source = Date.WeekOfMonth(#date(2025, 8, 24))
in
    Source      

The output of the above code is 5.

Example: First day of month.

Power Query M

let
    Source = Date.WeekOfMonth(#date(2025, 8, 1))
in
    Source  

The output of the above code is 1.