Since you have the handle off try this:

Heat up the fixture with a torch to try and loosen things up, maybe
some penetrating oil also. Probably a good idea to let the cold water
trickle out so you don't over heat that side.

A six point socket for the cartridge (probably stem & washer) then hit
the ratchet or breaker bar handle with a hammer. Don't try to muscle
it off, a sharp shock is less likely to break something than leaning on
a breaker bar.

If you get it apart:
Inspect the seat to make sure it is in good shape, if not a seat reamer
is an inexpensive tool that will clean it up.
When you are getting the new washer also get some packing string to
wrap the stem when it goes back together.

To take the entire faucet off there should be two big nuts on the
backside of the tub that you can't get to. If you have take the toilet
off that is no big deal as long as the floor and toilet flange are in
good shape.

Best advice I can give you is get it fixed before Sunday otherwise you
will have ten guys wearing ties in you bathroom prying on your pipes
*helping* you fix it.

Good luck!