Jason Morin wrote...
Probably because XL doesn't recognize it as a date. . . .
....

Testing is good.

=WEEKDAY("April 1, 2005")

returns 6 on my system. What does it return on yours?

However,

=WEEKDAY("April 1, 2005"&CHAR(160))

returns #VALUE! even though the argument would *APPEAR* the same as the
previous argument. So I think the safer bet is that the OP has stray
nonbreaking spaces in the date string, in which case

=WEEKDAY(SUBSTITUTE("April 1, 2005"&CHAR(160),CHAR(160)," "))

returns 6.