The reason for the difference is this:

Datename and Datepart are require dates, in this case they are being given integers, which are being converted to dates. 

7 is converted to 1900-01-08 00:00:00.000 (this happens to be a Monday), if you then take the datepart (day) of that its 8. 8 is converted to 1900-01-09 00:00:00.000, which happens to be a Tuesday.

Here is some code to show what I am saying.

declare @dtmTest datetime

declare @dtmTest2 datetime

set @dtmTest = 7

set @dtmTest2 = datepart(d, @dtmTest)

select @dtmTest, @dtmTest2, datename(dw, @dtmTest), datepart(d, @dtmTest), datename(dw, @dtmTest2)

You will get :

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

1900-01-08 00:00:00.000     1900-01-09 00:00:00.000     Monday                         8           Tuesday

(1 row(s) affected)

I hope this helps,

Chuck