create customer exit on 0calday

Hi ,

My requirement is i have to create customer exit variable on 0calday(characterstic). Default this value to current Monday of the week and popup to user.Validate User input with the Monday of the week.If its not a Monday give an error asking to enter Monday of that week

For ex: if i give date like 20.09.2007 it should display from starting of the week (i.e monday) to till that date.

PLz give me the instructions how to create

Thanks and Regards,

Madhavi