Following query will find the last day of the month. Query also take care of Leap Year.

Script:

DECLARE @date DATETIME
SET @date='2008-02-03'
SELECT DATEADD(dd, -DAY(DATEADD(m,1,@date)), DATEADD(m,1,@date))
            AS LastDayOfMonth
GO
DECLARE @date DATETIME
    SET @date='2007-02-03'
SELECT DATEADD(dd, -DAY(DATEADD(m,1,@date)), DATEADD(m,1,@date))
            AS LastDayOfMonth
GO

Following query will run respective to today’s date. It will return Last Day of Previous Month, First Day of Current Month, Today, Last Day of Previous Month and First Day of Next Month respective to current month.
DECLARE @mydate DATETIME
SELECT @mydate = GETDATE()
SELECT CONVERT(VARCHAR(25),DATEADD(dd,-(DAY(@mydate)),@mydate),101) ,
'Last Day of Previous Month'
UNION
SELECT CONVERT(VARCHAR(25),DATEADD(dd,-(DAY(@mydate)-1),@mydate),101) AS Date_Value,
'First Day of Current Month' AS Date_Type
UNION
SELECT CONVERT(VARCHAR(25),@mydate,101) AS Date_Value, 'Today' AS Date_Type
UNION
SELECT CONVERT(VARCHAR(25),DATEADD(dd,-(DAY(DATEADD(mm,1,@mydate))),DATEADD(mm,1,@mydate)),101) ,
'Last Day of Current Month'
UNION
SELECT CONVERT(VARCHAR(25),DATEADD(dd,-(DAY(DATEADD(mm,1,@mydate))-1),DATEADD(mm,1,@mydate)),101) ,
'First Day of Next Month'
GO