One of my favorite family traditions is making grasshoppers on Christmas Eve. My dad is the bartender. He combines equal parts creme de menthe (the green version, of course) and creme de cacao and then adds in vanilla ice cream and we have a party. How much liquor you add definitely depends on how strong you want it. It's pretty fun because you can tell how strong it is by how green it is. We like a nice minty green.
Does your family make a special cocktail for the holidays?