Sales-PW = 

VAR CurrentWeek =
    SELECTEDVALUE( 'Calendar'[WeekNo] )

VAR CurrentYear =
    SELECTEDVALUE( 'Calendar'[Year] )

VAR MaxWeekNo =
    CALCULATE(
        MAX ( 'Calendar'[WeekNo] ), 
            ALL ( 'Calendar' )
            )

RETURN
SUMX(
    FILTER( ALL ( 'Calendar' ),
        IF ( CurrentWeek = 1,
            'Calendar'[WeekNo] = MaxWeekNo && 'Calendar'[Year] = CurrentYear -1,
            'Calendar'[WeekNo] = CurrentWeek -1 && 'Calendar'[Year] = CurrentYear )
        ),
    [Total Sales]
    )

from time import strptime
month_name = 'Jan'
month_number = strptime(month_name, '%b').tm_mon
month_number

'''
%a  Locale’s abbreviated weekday name.    
%A  Locale’s full weekday name.    
%b  Locale’s abbreviated month name.  
%B  Locale’s full month name.      
%c  Locale’s appropriate date and time representation.    
%d  Day of the month as a decimal number [01,31].    
%f  Microsecond as a decimal number [0,999999], zero-padded on the left     (1)
%H  Hour (24-hour clock) as a decimal number [00,23].    
%I  Hour (12-hour clock) as a decimal number [01,12].    
%j  Day of the year as a decimal number [001,366].  
%m  Month as a decimal number [01,12].  
%M  Minute as a decimal number [00,59].      
%p  Locale’s equivalent of either AM or PM.   (2)
%S  Second as a decimal number [00,61].     (3)
%U  Week number of the year (Sunday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0.    (4)
%w  Weekday as a decimal number [0(Sunday),6].  
%W  Week number of the year (Monday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Monday are considered to be in week 0.    (4)
%x  Locale’s appropriate date representation.      
%X  Locale’s appropriate time representation.      
%y  Year without century as a decimal number [00,99].    
%Y  Year with century as a decimal number.  
%z  UTC offset in the form +HHMM or -HHMM (empty string if the the object is naive).    (5)
%Z  Time zone name (empty string if the object is naive).    
%%  A literal '%' character.
'''