How to find number of days since beginning of fiscal month?
This will do it:

DECLARE @d DATE = '20140502', @dm TINYINT = 26;
DECLARE @Start_Of_Fiscal_Month DATE

SET @Start_Of_Fiscal_Month = DATEADD(DAY, @dm-1, DATEADD(MONTH, -1 + DATEPART(dd, @d)/26, DATEADD(DAY, 1-DAY(@d), @d)));
SELECT @Start_Of_Fiscal_Month
SELECT DATEDIFF(dd, @Start_Of_Fiscal_Month, @d) + 1