Note that the output of your function depends on the value of @@DATEFIRST:

declare @DateFirst int

select @DateFirst = @@DATEFIRST

set datefirst 1

select dbo.Monday('2006-02-13','2006-02-20')  --Returns 2

set datefirst 7

select dbo.Monday('2006-02-13','2006-02-20')  -- Returns 1 (incorrect)

set datefirst @DateFirst -- Resets @@DATEFIRST to original value