When was the last time the Celtics didn't make the playoffs?

The last time the Boston Celtics did not make the playoffs was in the 2020-2021 NBA season. That year, the team finished with a record of 36 wins and 36 losses, placing them in the seventh seed, but they were eliminated in the play-in tournament. Prior to that, the Celtics last missed the playoffs in the 2006-2007 season.