you can use the second parameter of weekstart and weekend
from Qlik online help
WeekStart(date [, shift = 0 [,weekoffset = 0]])
Returns a value corresponding to a timestamp with the first millisecond of the first date ( Monday ) of the calendar week containing date. The default output format will be the DateFormat set in the script. Shift is an integer, where the value 0 indicates the week which contains date. Negative values in shift indicate preceding weeks and positive values indicate succeeding weeks. If you want to work with weeks not starting midnight between Sunday and Monday, indicate an offset in days in weekoffset. This may be given as a real number indicating days and/or fractions of a day.