Hi Trigger,
The @numDays parameter is the number of business days to move forward in time.
To get 2006-12-27, you should have used:
SET @Date = dbo.fnGetNextBusinessDay ('2006-12-24', 1)
to get the "first" next business day. Your use of:
SET @Date = dbo.fnGetNextBusinessDay ('2006-12-24', 10)
should get the 10th next business day.