Find First/Last Day Of Month For Given Date

DECLARE @entry_date DATETIME
	,@start_date DATETIME
	,@end_date DATETIME

SET @entry_date = '7-1-2013'
-- Set Start Date to first day of the month for given date
SET @start_date = (
		SELECT CONVERT(VARCHAR(25), DATEADD(dd, - (DAY(@entry_date) - 1), @entry_date), 101)
		)
-- Set End Date to last day of the month for given date
SET @end_date = (
		SELECT CONVERT(VARCHAR(25), DATEADD(dd, - (DAY(DATEADD(mm, 1, @entry_date))), DATEADD(mm, 1, @entry_date)), 101)
		)

----Last Day of Previous Month
SELECT DATEADD(s, - 1, DATEADD(mm, DATEDIFF(m, 0, GETDATE()), 0)) LastDay_PreviousMonth

----Last Day of Current Month
SELECT DATEADD(s, - 1, DATEADD(mm, DATEDIFF(m, 0, GETDATE()) + 1, 0)) LastDay_CurrentMonth

----Last Day of Next Month
SELECT DATEADD(s, - 1, DATEADD(mm, DATEDIFF(m, 0, GETDATE()) + 2, 0)) LastDay_NextMonth

--Last Day of Any Month and Year
DECLARE @dtDate DATETIME

SET @dtDate = '8/18/2007'

SELECT DATEADD(s, - 1, DATEADD(mm, DATEDIFF(m, 0, @dtDate) + 1, 0)) LastDay_AnyMonth

LastDay_PreviousMonth
-----------------------
2013-07-31 23:59:59.000

LastDay_CurrentMonth
-----------------------
2013-08-31 23:59:59.000

LastDay_NextMonth
-----------------------
2013-09-30 23:59:59.000

LastDay_AnyMonth
-----------------------
2007-08-31 23:59:59.000

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

07/31/2013                Last Day of Previous Month
08/01/2013                First Day of Current Month
08/07/2013                Today
08/31/2013                Last Day of Current Month
09/01/2013                First Day of Next Month

Source:

1. http://blog.sqlauthority.com/2007/05/13/sql-server-query-to-find-first-and-last-day-of-current-month/
2. http://blog.sqlauthority.com/2007/08/18/sql-server-find-last-day-of-any-month-current-previous-next/