Formal, i can not express this enough...
Formal means a floor length dress... or at least below the knee. Anything above the knee is not acceptable. You will be turned away at the door.
This is a classy event...
Guys are to wear tuxes or at least a suit and tie.
Some exceptions will be made on dresses. If it is at least knee length, thats acceptable.
I'm sorry to sound so strict.