Hello comunity

I need some help for create a procedure or function that can use on any code to return by Year the number of days in determinate Month.
For example if i want to know for a specific year the number of days in determinate month.
i can create for example sails between two dates and for a particular year and i want to return the totals of sails in this year by divide them for the number of days in this month.

Many thanks
Luis Santos