I'm going to disagree about making it ahead of time. The best part of green bean casserole is the crunchy onions on top and how they contrast with the creamy beans. If you make it ahead of time, the onions will be nasty and soggy. It only takes 2 minutes to stir together, and any time you might save by baking it part way (before you add the onions on top) will be lost by chilling/reheating it the next day.
When I am assigned this dish for thanksgiving - I cook it part way at home, drive the 15 min. to MIL's house, and then finish it in her oven with the onions.