As they said on Bungie.net, the Halo 3:ODST Firefight level, Crater (Night), is the drop site of Dare, the beautiful Captain of the Halo 3: ODST squad. I clearly remember Bungie stating that all the Flashbacks take place in the day. This gives me two theories, one, is that almost (or possibly every) Firefight map, is based off of a campaign piece. and my other theory is, that the reason it says night, may be, because in the level selection, Crater, may be a level in campaign, but Crater (Night) may be a Firefight level. And Bungie's answer, Yes, and yes, to the following question, backs my theory up.
Q. Any super secret stuff on the map that we can’t talk about yet that will alter the way players will approach combat? Doesn’t the parenthetical “Night” suggest that there is, in fact, a Crater somewhere that exists at some other time-of-day?
Bungie's answer, was, in fact,
A. Yes and yes. :-)