WEEKDAY Returns a number from 1 to 7 identifying the day of the week of a date. By default the day ranges from 1 (Sunday) to 7 (Saturday).
Syntax
WEEKDAY(date, return_type)
Parameters
date – A date in datetime format.
Dates should be entered by using the DATE function, by using expressions that result in a date, or as the result of other formulas.
return_type – A number that determines the Return value:
Return type: 1, week begins on Sunday (1) and ends on Saturday (7). numbered 1 through 7.
Return type: 2, week begins on Monday (1) and ends on Sunday (7).
Return type: 3, week begins on Monday (0) and ends on Sunday (6).numbered 1 through 7.
Return Value
An integer number from 1 to 7.
Reference Data
https://docs.microsoft.com/en-us/dax/dax-function-reference