Custom Pay Periods in Date Table

-- I can't access the pbix file due to
-- the company policy. Sorry. But here's
-- something that should get you started...

[Pay Period Number] = -- calculated column
var __currentMonth = Dates[MonthNumber] -- month number 1,2,...,12
var __currentYear = Dates[Year] -- obvious
var __currentDate = Dates[Date]
var __numOfMondaysInMonth =
	COUNTROWS(
		filter(
			Dates,
			Dates[MonthNumber] = __currentMonth
			&& Dates[Year] = __currentYear
			&& Dates[DayName] = "Monday" -- you should have this field in your Dates
		)
	)
var __isMonday = ( Dates[DayName] = "Monday" )
var __whichMonday =
	COUNTROWS(
		filter(
			Dates,
			Dates[MonthNumber] = __currentMonth
			&& Dates[Year] = __currentYear
			&& Dates[DayName] = "Monday"
			&& Dates[Date] <= __currentDate
		)
	)
var __result =
	if (
		__isMonday,
		
		SWITCH( TRUE(),
		
			__numOfMondaysInMonth = 4,
				SWITCH( __whichMonday,
					2, "2nd",
					4, "4th"
				) & " Pay Period",
				
			__numOfMondaysInMonth = 5,
				SWITCH( __whichMonday,
					1, "1st",
					2, "2nd",
					5, "5th"
				) & " Pay Period",
			
			"Check out the calendar for errors."
		)
	)
return
	__result

Best

Darek