AddDays(AddMonths(MakeDate(1, [Month], [Year]), 1), -1)
MakeDate(1, [Month], [Year]), 1) : This makes a date of the first day of whatever month/year you pass as parameter.
AddMonths(MakeDate(1, [Month], [Year]), 1) : Adds 1 month so it is the first day of the next month...