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T-SQL 359 Characters
CREATE PROCEDURE f13( @d Date=null)AS
SET @d=ISNULL(@d,GETDATE())
DECLARE @n Date
SET @n='0001-01-13'
SET @n=DATEADD(YY,YEAR(@d)-1,@n)
SET @n=DATEADD(M,MONTH(@d)-CASE WHEN DATEPART(dd,@d)>12 THEN 0 ELSE 1 END,@n)
;WITH d AS
(SELECT @n d,DATEPART(dw,@n) y
UNION ALL SELECT ...