percentage of completion of the month = 
CALCULATE (
    DIVIDE ( DAY ( TODAY () ), DAY ( LASTDATE ( 'Date'[Date] ) ), 0 ),
    FILTER (
        ALL ( 'Date' ),
        YEAR ( 'Date'[Date] ) = YEAR ( TODAY () )
            && MONTH ( 'Date'[Date] ) = MONTH ( TODAY () )
    )
)