Of course no.of the days in the month. If you based on 26 days, the total is surely different.
If the employer salary is RM300 per month. Let say on April, he took 1 day unpaid leave. That's means the working days is 29 days.
If you based calculation of 26days, it will be like this:-
RM300 ÷ 26 x 29 working days = RM334.61 salary for that month
If you based on the no.of the days in the month, you'll get:-
RM 300 ÷ 30 x 29working days = RM290.00 salary for that month.
If your company choose the 1st calculation, I rather working there and take unpaid leave every month.