Between knowing that day "0" is the first of January, 1900 and that day was a Monday along with a little integer math and a little direct date math, it can be greatly simplified.
DECLARE @Today DATETIME = '20180417'
;
SELECT *
FROM dbo.TESTBUILDS
WHERE [Build Day] >= DATEADD(dd,DATEDIFF(dd,0,@Today)/7*7,7)
AND [Build Day] < DATEADD(dd,DATEDIFF(dd,0,@Today)/7*7,14)
;
Just substitute GETDATE() for @Today in the formulas and forget the DECLARE, which was included just to demonstrate.
--Jeff Moden
Change is inevitable... Change for the better is not.