Look at the explanation of DATENAME. As you are supplying @@datefirst as your date, SQL Server assumes that you are looking for the values of day 7 according to SQL Server date arithmetic, which is 08/01/1900

select

dateadd(dd,7,0) as 'Datefirst'

, datename(dw,7) as 'WD Name'

, datename(dw,datepart(d,7)) as 'DP WD Name'

Datefirst WD Name DP WD Name

------------------------------------------------------ ------------------------------ ------------------------------

1900-01-08 00:00:00.000 Montag Dienstag

(1 row(s) affected)