Twenty years from now you’ll say you were there. Just like the millions who claim to have been at Fenway when Carlton Fisk hit THE HOME RUN in 1975.

Not only will you not have switched channels when the Celtics fell behind by 20-something, but you were among the 18,000 at the FleetCenter.

You were there when Antoine Walker screamed at his teammates during the timeout at the end of the third quarter. You were there when Rodney Rogers made six in a row from the free-throw line. You were there.

The advice around here is to wait and watch the fourth quarter of an NBA game. No need to watch the first three quarters. It’s like watching the opening act at a Creed concert. You can’t remember who they were once you’ve heard “My Sacrifice.”

Unfortunately, yours truly wasn’t at the FleetCenter. Fresh off a couple of hours of whacking weeds and cutting grass, the recliner beckoned. Ignoring the aforementioned time-tested NBA axiom, the TV came on, Hannah Storm eventually stopped talking and soon after the opening tip-off, the Celtics stopped playing.

It was disgusting really.

Ugly.

They threw it away. They kicked it away. They missed shots Brewer PAL players could be trusted to make. And it was a team effort. No one seemed to care. No one hustled. No one got angry.

If you were like me and not at the FleetCenter, we had to rely on NBC floor reporter Lewis Johnson for information. Celtics coach Jim O’Brien is telling his players they have to tighten their defense, Johnson reported.

But they didn’t. They played the same disinterested style of basketball they played two years ago. In fact, it wouldn’t have been very surprising if the camera had caught Rick Pitino pacing in front of the Celtics’ bench, shouting out instructions to players who had long ago stopped listening to him.

So, with the Celtics trailing by 20-plus, yours truly did the only thing a sensible person would do. I poured a glass of soda and switched channels to HBO and “A Knight’s Tale” (joined in progress). I traded a fake basketball team for a fake knight. The knight was more enjoyable (Paul Bettany was a riot playing Geoffrey Chaucer).

Still, curiosity lingered. A flick of the clicker and the FleetCenter was a madhouse. NBC’s Mike Breen was shouting that the Celtics had opened the fourth quarter with an 11-0 run. And Larry, Kevin and Robert hadn’t walked through that door to ignite the comeback.

It seems that while I was off in never-never land, Walker had lashed out at his teammates. He evidently told them he was embarrassed and that they should be embarrassed and that maybe they should start playing a little.

The replay showed Walker looking directly at Paul Pierce and giving him a verbal beating.

Pierce responded. He took the ball to the basket. He repeatedly traveled and it wasn’t called. As someone pointed out, when a player arrives at superstar status he can shuffle his feet, walk or run without it being called, except in the most obvious cases. (The truth be told, walking is rarely called in the NBA. Even on the most insignificant players.)

Rodney Rogers and Kenny Anderson hit free throws and suddenly it was 1986. The FleetCenter was rocking.

Fans were screaming. Leprechauns were dancing. The Nets were staggering. The ball was taking funny bounces. Kerry Kittles tripped over his own feet and tossed a gift down court that Kenny Anderson ran down for an easy basket.

And it was over. Walker jumped up onto the scorer’s table. Jo Jo White danced on the parquet, a huge smile on his face.

The New York Post headline the following day was “Choke of the Century.” Greg Norman and Jean Van de Velde will be eternally grateful.

Of course, you know this. You were there. Twenty years from now you can tell anyone who will listen, you were there.