returns the current value, for a session, of SET DATE FIRST, i.e it returns first day of week, TinyInt Value from 1 through 7 representing Monday to Sunday as shown Below.

1 for Monday
2 for Tuesday
3 for Wednesday
4 for Thursday
5 for Friday
6 for Saturday
7 for Sunday

For Example, If the first day of week is Sunday (US) , Then @@DateFirst would return 7.

If the first day of week is Monday (Europe), Then @@DateFirst Would return 1.

SET DateFirst = 3

The above statement would set Wednesday as first day of the week for the current Session.

‘July 1st 2009 is a wednesday

SET DateFirst = 1 -- or SET LANGUAGE Italian;
Select DATEPART(dw, '20090701')

would return 3 (since Wednesday is 3rd day from Monday)

SET DateFirst = 7 -- or SET LANGUAGE us_english;
Select DATEPART(dw, '20090701')

would return 4 (since Wednesday is 4th day from Sunday)

DATEPART return the weekday according @@DateFirst.