CREATE  FUNCTION [GetLastDateofMonth] 
(@Date as DateTime)  

RETURNS DateTime AS  
BEGIN 

Declare @LastDate DateTime 
Set @LastDate = DateAdd(Month, 1, @Date - Day(@Date) + 1) -1

RETURN @LastDate

END
GO

http://www.databasejournal.com/scripts/article.php/3583671/Get-FirstLast-Date-of-Month.htm

Get last date of a month

sql

SQL