BOSTON — This is how it used to be.

Big game.

A pulsating Garden, alive with cheers and energy.

A postseason worth caring about.

NBA playoffs worth caring about.

This is how it used to be, back when the Celtics were NBA royalty, all those championship banners hanging from the rafters, their symbolic signature. Back when the Big Three were Bird, McHale and Parish. Danny Ainge was the guy throwing jump shots from the backcourt and not the boss, and big games in May and June were part of the schedule.

But a strange thing happened to the Celtics the past decade or so.

They started not to matter.

Not in any one dramatic moment, certainly. Not with any formal announcement. But gradually, over time, a slow descent, the result of too many seasons that went nowhere, too many seasons that ended long before the big games even began. All of this happening in a time when Boston became the unofficial city of champions, both the Red Sox and the Patriots the kings of their respective sports.

Is it any wonder that the Celtics seemed to become more and more irrelevant?

The seasons continued to slide by and the Celtics seemed more and more about the past, almost as if their glory days had become a burden, a constant reminder that once upon a time things were so different.

Sure, there were a few bright spots on the bleak canvas. Rick Pitino was hired amid great fanfare in 1997. The team actually got to the Eastern Conference finals in 2002. In retrospect, though, those were like blips on the radar screen.

After that?

More of the same.

The sense it was all slipping away, the glory days receding further in the distance, the Celtics little more of an afterthought in a new Boston sports arena that seemed to go from the Patriots in the playoffs to the Sox in spring training.

Not surprising. You almost had to be approaching middle age to even remember it. There was an entire generation coming along for whom Larry Bird was about as far removed as Paul Revere, just another musty name from the past, a figure on some old film clip somewhere. To them, all those championship banners hanging in the rafters were like old pictures in a museum, like artifacts from a different time.

Even with the new ownership and the hiring of Ainge as director of basketball operations and a new rebuilding cycle, it all seemed like just another chapter in the same old story.

Remember Raef LaFrentz and Marcus Banks?

Remember Rodney Rogers and Tony Delk, and so many others who came and went in a revolving door of mediocrity?

These were the Celtics in all those lost years, all about promise and some undetermined future, all about players coming and players going, a franchise forever trapped on some treadmill of mediocrity and rebuilding. A franchise that always seemed to be going home for the summer before the big games started.

Until now.

That’s the thing to remember as this series with the Pistons plays out.

The legacy of this team is not how far it goes in these playoffs. Not really. The legacy of this season is that people now care about the Celtics in ways they haven’t in a long time. They watch the games. They talk about them. They call talk shows about them. The Celtics control so much of the conversation in ways they haven’t in such a long time.

That began, of course, last summer when the blockbuster trades that brought both Ray Allen and Kevin Garnett here changed everything. The groundswell started, and has never let up. The regular season was more of the same. Big crowds. The Garden a homage to this team. Eventually the best record in the NBA.

Who could have made that up last spring when the big hope was that the Celtics could get the first pick in the NBA Draft?

Who could have made it up that this spring the Celtics would be one of the last four teams left in the NBA playoffs, their first trip back to the Fnals in more than two decades.

Last night was a reaffirmation of this.

Game Five of the Eastern Conference finals, and all the tumult that came with that, the eyes of the basketball world on the Garden in Boston, just the way it used to be. Game Five of the Eastern Conference finals, in the middle of the basketball beast. The Celtics right in the middle of the sports storm, the way they used to be.

breynold@projo.com