create function dbo.tvf_EOM (
    @iDate datetime
)
returns table
as
    return select
                dateadd(mm, datediff(mm, 0, @iDate) + 1, -1) as EndOfThisMonth,
                datename(dw,dateadd(mm, datediff(mm, 0, @iDate) + 1, -1)) as DatenameEndOfThisMonth;
go

select
    dateadd(dd, t.N - 1, 0) as TheDate,
    EndOfThisMonth,
    DatenameEndOfThisMonth
from
    dbo.Tally t
    cross apply dbo.tvf_EOM(dateadd(dd, t.N - 1, 0))
where
    t.N between datediff(dd, 0, getdate()) and datediff(dd, 0, getdate()) + 120;

--===== Create and populate a 1,000,000 row test table.
     -- Column "RowNum" has a range of 1 to 100,000 unique numbers
     -- Column "SomeInt" has a range of 1 to 50,000 non-unique numbers
     -- Column "SomeLetters2" has a range of "AA" to "ZZ" non-unique 2 character strings
     -- Column "SomeMoney has a range of 0.0000 to 99.9999 non-unique numbers
     -- Column "SomeDate" has a range of  >=01/01/2000 and <01/01/2010 non-unique date/times
     -- Column "SomeCSV" contains 'Part01,Part02,Part03,Part04,Part05,Part06,Part07,Part08,Part09,Part10'
     --        for all rows.
     -- Column "SomeHex12" contains 12 random hex characters (ie, 0-9,A-F)
     -- Jeff Moden
 SELECT TOP 1000000
        SomeID       = IDENTITY(INT,1,1),
        SomeInt      = ABS(CHECKSUM(NEWID()))%50000+1,
        SomeLetters2 = CHAR(ABS(CHECKSUM(NEWID()))%26+65)
                     + CHAR(ABS(CHECKSUM(NEWID()))%26+65),
        SomeCSV      = CAST('Part01,Part02,Part03,Part04,Part05,Part06,Part07,Part08,Part09,Part10' AS VARCHAR(80)),
        SomeMoney    = CAST(ABS(CHECKSUM(NEWID()))%10000 /100.0 AS MONEY),
        SomeDate     = CAST(RAND(CHECKSUM(NEWID()))*3653.0+36524.0 AS DATETIME),
        SomeHex12    = RIGHT(NEWID(),12)
   INTO #JBMTest
   FROM Master.dbo.SysColumns t1
  CROSS JOIN Master.dbo.SysColumns t2
&#160;
--===== Add a primary key just because
  ALTER TABLE #JBMTest
        ADD PRIMARY KEY CLUSTERED (SomeID)

 CREATE FUNCTION dbo.LastDayOfMonth (@AnyDate DATETIME)
RETURNS DATETIME
     AS 
  BEGIN
 RETURN DATEADD(mm,DATEDIFF(mm,-1,@AnyDate),-1)
    END

    SET NOCOUNT ON
DECLARE @BitBucket DATETIME
 
  PRINT '========== Inline method =========='
    SET STATISTICS TIME ON
 SELECT @BitBucket = DATEADD(mm,DATEDIFF(mm,-1,SomeDate),-1)
   FROM #JBMTest
    SET STATISTICS TIME OFF
 
  PRINT '========== UDF method =========='
    SET STATISTICS TIME ON
 SELECT @BitBucket = dbo.LastDayOfMonth(SomeDate)
   FROM #JBMTest
    SET STATISTICS TIME OFF

========== Inline method ==========
 
SQL Server Execution Times:
   CPU time = 1172 ms,  elapsed time = 4311 ms.
========== UDF method ==========
 
SQL Server Execution Times:
   CPU time = 80047 ms,  elapsed time = 147857 ms.