Dear Saraiya,
The term " month " has several meanings.
As a stretch of a period of time from the perspective of an year, it is one of the twelve periods into which an year is divided and therefore each would be having 28 or 29 to 31 days.
In Commerce, it is a period of 30 successive days.
As unit of time based on the revolution of the Moon around the Earth, it is the period between two successive new moons or on average 29.53 days.
The length or duration of a month, therefore, is a matter of approximation for the purpose of convenience of calculation.
As per your post your payment is for a month comprising of 26 days i.e., 26 working days plus 4 holidays in a month notionally comprising of 30 days on average. In other words the monthly wages/salary paid for 26 working days includes the 4 weekly off days. Since a week is cosidered to comprise of 7 days in terms of days, the principle of 7th successive day after 6 days of continuous work as the weekly-off has emerged.
The universal practice of calculation of over time work is based on actual hours worked in excess of normal working hours, isn't it? So, compensation for OT work is determined on the basis of hourly rate only and as such the actual no of days in a particular month, in my opinion, is immaterial for the purpose of calculation of OT Wages.