It depends from company to company is all that I would like to suggest. However,
As per the laws, there should be 1 rest day after a maximum of 6 days of work in a week. So by default, Sunday becomes a paid holiday.
Most companies practice a 30-day process irrespective of the months having 31 or 28/29 days.
Many companies take the number of days in the month like 31 for jan, 28/29 for feb, 31 for march, 30 for april and so on.
30-days feature
You do not have to change the formula every month depending on the no. of days in the month.
The salary per day for any month would be the same and hence calculation becomes easy.
Monthly days feature:
Though the no. of days in a month changes from month to month basis, and hence salary per day changes month on month.
Salary per day for the month of the feb is the highest as it has the lowest number of days in the month.
Salary per day for rest of months have minor changes because of 30/31 days.
Best part is that you tell your employees that all the sundays and the holidays are paid ones.
I hope this was able to clear you and give you better ideas on which approach to consider.
All the best.