Buffalo airport is open at all times, as described here.

However just because the operating side is open does not mean the terminal building has to be. If you turn up early for the first flight of the day you may find the building locked, although there are usually security guards around to keep an eye on things. Airports with first flights at 06.00 often open up the building around 04.00. There are other aspects that may have restricted hours; customs for example may only be available for limited hours during an airport's operations, which restrict its ability to handle international flights. There are reports of some surprisingly significant airports in the US where overnight international flights arriving early have to sit with all the passengers on board until the customs officers turn up for work.

I am sure my good friend stut is aware that this was a planning condition imposed at the time of building the airport, it has to be closed from 12.30 Saturday to 12.30 Sunday. As it would have little business at those times anyway, it's no great loss. Likewise it also cannot open until 09.00 on public holidays, so early flights have to be rescheduled on those days. Other airports have other conditions imposed on them by their local communities.