Re: DAX first day of current month

My suggestion:

Previous month score for top scorer = 
	VAR TopScorers = TOPN( 1, VALUES( Scores[name] ), CALCULATE(AVERAGE(Scores[Score])) )
        VAR PrevMonth = CALCULATETABLE( DATEADD(Scores[Date],-1, MONTH), LASTDATE( Scores[Date]) )
		  RETURN
CALCULATE( AVERAGE(Scores[Score])
		, TopScorers
		, PrevMonth
	)

Note the TOPN function wil return several players if you have ties in your current selection. Hence the average calculation.

Also, in case several months/dates are selected, I take the last one and calculate the previous month for this date.