Dear Shweta,
Universally, Sundays are considered holidays. So, in a month, all other days are working days. Most months will have only four Sundays. But the worst-case scenario is when a month has five Sundays since more than five is not possible. Therefore, the number of working days should be the worst-case total number of days in a month minus the 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 30-day months than 31-day months, as well as the 28/29 days in the 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.