The week before week 1 of course
Week 0 I believe is relatively new, like debuted in the last 3 years or so and this is I believe the first P5 vs P5 matchup. Usually the "marquee" game is something like Washington State vs Utah State.
This game was given an exemption by the NCAA because they're having a day-long celebration of the 150th season of college football. Or something. There was another one a few years ago when they played
The game later tonight though, is a originally rule the NCAA put in to help out Hawaii (and should anyone start a program up there, Alaska), who are allowed to play an extra regular season game - along with anyone else who go out there to play them, if they choose. The only one who took the extra game this year is Army. That's not new though. Hawaii used play the extra game in December, but with every conference having a championship game now, that's no longer an option, thus Week 0 came to be. Anyone who plays at Hawaii can schedule a game against another team who also plays at Hawaii in Week 0. Wyoming and Colorado State did this last year, and Stanford did it at least once.
There's also an FCS exemption if the game is nationally televised on TV or cable. No internet-only.