Getting the weekday name

[Peter Haworth]

I'm using the datetimetoJulian function in Sarah's great datetime  
stack as part of a way to get the alpha weekday name.  The words in  
Sarah's stack say that it's possible to get the day number of the week  
by taking the remainder of dividing the Julian days by 7 but when I do  
that, I consistently get a day number that is 2 less than it should be  
- that's assuming of course that day 1 would be Sunday.

I can always just add 2 to the day number but wondering if anyone  
knows why this would happen?

Pete Haworth