Try this variation on my formula and see if it works (I **think** the
"aaaa"
returns the day name with the localized spelling for the computer it is
being run on)...

=IF(DATE($B$5,$B$4,$B9)<=DATE($B$5,$B$4+1,0),TEX T(WEEKDAY(DATE($B$5,$B$4,$B9)),"aaaa"),"n/a")
...

May work for English and German (and other) regional settings, but I
wonder whether it'd fail for French and presumably other romance
language regional setting where 'a' stands for year.

Internationalization is a PITA.

That is why I'm glad that in my entire programming career, I have never had
to deal with Internalization issues... I have seen some posting regarding
various treatments required to account for it in compiled VB and it looked
like a horrible thing to have to try and deal with properly. As for the 'a'
for year issue, I guess we'll have to wait for someone with French Excel to
try it out.

By the way, for those still reading this thread... the "aaa" and "aaaa" also
works in the formatting patterns for Custom Formatting cells (with the
question about French Excel still pending).

Rick