How to check if datetime happens to be Saturday or Sunday in SQL Server 2008

Many ways to do this, you can use DATENAME and check for the actual strings 'Saturday' or 'Sunday'

SELECT DATENAME(DW, GETDATE())

Or use the day of the week and check for 1 (Sunday) or 7 (Saturday)

SELECT DATEPART(DW, GETDATE())

This expression

SELECT (((DATEPART(DW, @my_date_var) - 1 ) + @@DATEFIRST ) % 7)

will always return a number between 0 and 6 where

0 -> Sunday
1 -> Monday
2 -> Tuesday
3 -> Wednesday
4 -> Thursday
5 -> Friday
6 -> Saturday

Independently from @@DATEFIRST

So a weekend day is tested like this

SELECT (CASE
           WHEN (((DATEPART(DW, @my_date_var) - 1 ) + @@DATEFIRST ) % 7) IN (0,6)
           THEN 1
           ELSE 0
       END) AS is_weekend_day