There is a thin paperback illustrated children's book I used to have
that I can not remember the name of.  In the story, it is a girl's
birthday, but she thinks that everyone has forgotten it.  At the end
they have thrown her a surprise party.  The illustrations portray the
backgrounds as mysterious and sometimes a bit grotesque.  I think I
also remember her outside of a closed store front window, really
wanting something inside.  Can you tell me what book this is and who
the illustrator was?

I wonder if you may be remembering "Something Queer at the Birthday
Party," written by Elizabeth Levy and illustrated by Mordecai
Gerstein.

"Kids will love playing detective in this rollicking, sunny story of
vanishing paintings and exciting treasure hunts, and will delight in
Gerstein's eccentric and lovable illustrations."

http://www.alibris.com/search/search.cfm?qwork=6200847&wtit=something%20queer%20at%20the%20birthday%20party&matches=11&qsort=r&cm_re=works*listing*title

Thanks for trying, but I don't think that was it.  I'm pretty sure the
title had the girl's name in it.  The mood of the illustrations was
much more morbid, and I would never call it a "sunny story".  Also,
the ending was a surprise, so the fact that there was a birthday party
at all was kept a secret, especially from the title.

how long ago did you have the book? was it around in the 70's,80's, 90's?

It was the 80s.