Dear Shweta,
Universally sundays are considered holidays. So in a month, all other days are working days. Now most months will have only 4 sundays. But the worst case scenario is when a month has 5 sundays since more than 5 is not possible. So the number of working days should be the worst case total no. of days in a month - worst case number of sundays. That will be 31-5 which is equal to 26. On pro-employee / humanitarian grounds, facts such as there are more of 30-day months than 31-day months as well as the 28/29 month of February, are ignored.
I'm guessing that in a year one gets 26 working days as leave based on the same logic.
Hope that helps.
Regards,
Mathew.