Words are fun
So there's "bank" in the earthworks sense: a mounding of earth against another feature. (River banks are a ledge created by the river.) So you build an earthworks wall and a little ledge to stand on to look or shoot over the wall. What do you name this? A "banquette", a little bank. Say you create a bench all along the wall of a room. It's not earthworks, but it looks about the same; it also is a "banquette". A table of similar location and shape subsequently may also be named a "banquette table".
Now, when you're eating a large meal in a room full of "banquettes", there may be a light course where you sit on the bench with a snack and chat and rest. This is the "banquet" course. This characteristic of a certain type of feast may come to be confused with the whole feast, probably by people only passingly familiar with it: thus the general meaning is reversed from its technical meaning, and eventually any large, fancy feast becomes a "banquet".
Curiously, "bank" in the financial institution sense is backwards from our beginning above. The earthworks "bank" probably comes from analogy to a small table, the Latin for bench or small table being like to "bank" (banco). Money changers and lenders had a characteristic table they worked from, and became known by their apparatus as "bankers". This may also be the origin of "bankrupt": when you lose all your money and go out of business they break your table.
This is not a real etymology, but the original words seem to be preserved closely enough in English to still make sense. And it's also not strictly researched, so please don't cite it, but I think it's close. See http://www.etymonline.com to do your own research. A copy of the OED would also help.