Hello Nishant,
You won't be able to calculate a Monday of a given date in a report formula column, because some functions are not allowed there due to performance reasons. My suggestion is to use a custom field. The following formula should return Monday of the week Date is in:
Date - Days(Case(ToString(Date, "ddd"), "Mon", 0, "Tue", 1, "Wed", 2, "Thu", 3, "Fri", 4, "Sat", 5, 6))
Please let me know if this helps.
Best regards,
Roland