The entire 2005 season.
Hurricane Katrina hit a week and a half before Week 1, and the Saints bolted to San Antonio. Owner Tom Benson tried his damndest to keep the team there and San Antonio government officials didn't do a very good job of hiding there desire to keep the Saints while New Orleans was at its lowest.
The team played 4 games at LSU, and sold 60,000+ tickets for the first. Then, after the game, Benson accused Saints fans of trying to kill him on the way out of Tiger Stadium, and said he would not come back to Louisiana (a local news camera had footage of Benson leaving, and he was never threatened -- in fact he attacked the camera man). Anyway, after that first game, there was no more than 20,000-30,000 fans for the next three games in Tiger Stadium.
All the talking heads in the national media were saying how the Saints would never play in New Orleans again, and pretty much everyone around here, including local media, had kind of come to the realization that would happen, especially with the Superdome taking on extensive damage and Benson firing one of the few people in the front office that confronted him about trying to keep the team in San Antonio.
For all the concern of rebuilding the city, the thought of the Saints not coming back made for some pretty depressing times in south Louisiana.
Paul Tagliabue stepped in, though, forced Benson to come back to New Orleans, and since then, there's a 35,000+ waiting list for season tickets. And the team just reached an agreement with the state to stay through at least 2025.