What happens if your initial FROMdate or TOdate happen to be a sunday? This batch shows solutions either way:

-- Recalculating startdate

;WITH SampleData AS (

SELECT TheDate = CAST('06 Jan 2013' AS DATE) UNION ALL

SELECT TheDate = '07 Jan 2013' UNION ALL -- StartDate

SELECT TheDate = '08 Jan 2013' UNION ALL

SELECT TheDate = '09 Jan 2013' UNION ALL

SELECT TheDate = '10 Jan 2013' UNION ALL

SELECT TheDate = '11 Jan 2013' UNION ALL

SELECT TheDate = '12 Jan 2013' UNION ALL

SELECT TheDate = '13 Jan 2013' UNION ALL

SELECT TheDate = '14 Jan 2013' UNION ALL

SELECT TheDate = '15 Jan 2013' UNION ALL

SELECT TheDate = '16 Jan 2013' UNION ALL

SELECT TheDate = '17 Jan 2013' UNION ALL

SELECT TheDate = '18 Jan 2013' UNION ALL

SELECT TheDate = '19 Jan 2013' UNION ALL

SELECT TheDate = '20 Jan 2013' UNION ALL

SELECT TheDate = '21 Jan 2013'

)

SELECT

DATENAME(weekday,TheDate),

TheDate,

FirstSundayAfterTheDate1 = DATEADD(dd,6-DATEDIFF(dd,7,TheDate)%7,TheDate), -- use this

FirstSundayAfterTheDate2 = DATEADD(dd,7-DATEDIFF(dd,6,TheDate)%7,TheDate) -- or this

FROM SampleData

CROSS APPLY ( -- workings - helps to explain the solution

SELECT

calc1 = DATEDIFF(dd,7,TheDate),

calc2 = 6-DATEDIFF(dd,7,TheDate)%7

) x

-- Recalculating enddate

;WITH SampleData AS (

SELECT TheDate = CAST('06 Mar 2013' AS DATE) UNION ALL

SELECT TheDate = '07 Mar 2013' UNION ALL

SELECT TheDate = '08 Mar 2013' UNION ALL

SELECT TheDate = '09 Mar 2013' UNION ALL

SELECT TheDate = '10 Mar 2013' UNION ALL

SELECT TheDate = '11 Mar 2013' UNION ALL

SELECT TheDate = '12 Mar 2013' UNION ALL

SELECT TheDate = '13 Mar 2013' UNION ALL

SELECT TheDate = '14 Mar 2013' UNION ALL

SELECT TheDate = '15 Mar 2013' UNION ALL

SELECT TheDate = '16 Mar 2013' UNION ALL

SELECT TheDate = '17 Mar 2013' UNION ALL

SELECT TheDate = '18 Mar 2013' UNION ALL -- enddate

SELECT TheDate = '19 Mar 2013' UNION ALL

SELECT TheDate = '20 Mar 2013' UNION ALL

SELECT TheDate = '21 Mar 2013'

)

SELECT

DATENAME(weekday,TheDate),

TheDate,

calc1 = DATEDIFF(dd,6,TheDate),

calc2 = DATEDIFF(dd,6,TheDate)%7,

LastSundayBeforeEnddate1 = DATEADD(dd,0-DATEDIFF(dd,6,TheDate)%7,TheDate), -- use this

LastSundayBeforeEnddate2 = DATEADD(dd,-1-DATEDIFF(dd,7,TheDate)%7,TheDate) -- or this

FROM SampleData