calendar days for particular month in day

Question

  
 working days textbox datetimepicker(calendar)

 when i click the datetimepicker that particular month of day will be displayed i done through coding.

 then i have net payable days  textbox if i click the particular month that month of day sundays to be subracted and remaining days to be displayed in the net payable days textbox.

for your understand

  working days  textbox datetimepicker(calendar)     net payable days  textbox   26
                              31    if i choose january 31                 26 days to be dispalyed
                                     to be dispalyed in textbox