It seemed like a good sign when the Celtics beat the Cavaliers in Cleveland on Opening Night. Their 6-0 start was promising, as was their 23-5 record following a Christmas Day win over the Magic.

But four months later, they enter the playoffs with 50 wins and the fourth seed in the Eastern Conference.

A .500 record since Dec. 25? What happened?

“The feeling, I don’t know, I really can’t put my finger on it beside that it was something with our team that was different,” said Glen Davis.

Although it is nearly impossible to identify one specific event over the course of an 82-game season, the fact of that matter is that things changed with the Celtics. They didn’t defend their homecourt, they blundered away late leads, and they simply lost games that they should have won (think Nets).

“I think we got off to a great start,” said Rajon Rondo. “But other than that, things just, I wouldn’t put a pinpoint on one particular thing but just things happen — injuries, communication failures, not playing together, not playing well, a mixture of all those each game, not finishing games. But it’s behind us. We’ve got another season under our belt. Most teams would be happy with 50 wins, but it’s more pressure, obviously, but it’s all about 16 wins now.”

Now the Celtics are focusing on looking ahead to the playoffs, a “new” season in which they are 0-0. But in order to find success in upcoming games, they will have to right their wrongs from those which they lost.

During a season in which Kevin Garnett and Paul Pierce, among others, missed significant time, injuries had an obvious impact on the team. However this team has battled through injuries before — the C's made it to the Eastern Conference semifinals last season without Garnett — and never made excuses when they were shorthanded.

A red flag actually was raised with the starting five on the court. The Celtics were 20-4 (8-3 at home with losses only against plus-.500 teams) when they took on the 6-19 76ers on Dec. 18 in Boston. The C’s led by as many as 15 in the third quarter but, in an unexpected turn, slowly let the Sixers back in the game. They were outscored 56-44 in the second half before Elton Brand stole the game with 7.7 seconds left.

“It was around that time, we played Memphis [the previous game] in Memphis and noticed, like, man, we’re not really playing good right now,” said Marquis Daniels. “Just, I don’t know. It’s hard to say. It’s hard to point out. Like I said, we just really weren’t doing a lot of things well. … [But] you can’t put it on that because we could have bounced back and gone the opposite way. But it’s been a roller coaster year for us, pretty much for every guy on this team.”

The following month, the Celtics surrendered a 50-41 halftime lead over the Mavericks in Boston and lost, 99-90. They were outscored 34-18 in the third quarter as they dropped their third straight home game.

Less than two weeks later, the C’s blew a 16-point advantage over the Magic in Orlando and were outscored 35-22 in the fourth. Rashard Lewis dealt the final blow with a basket with 1.3 seconds to go in the 96-94 loss.

The Celtics dropped the next two games — a loss to the Hawks and a one-point defeat to the Lakers. They were up by as many as 11 in the fourth against the Lakers before Kobe Bryant hit the game-winner with 7.3 seconds remaining.

A single loss didn’t cause a season of ups and downs. Neither did a handful of defeats. But the culmination did make an impact.

“The first couple of times, given, you’re going to lose a couple of games, but then all of a sudden it became almost like a pattern," said Shelden Williams. "People started hanging heads and stuff and it was kind of hard to get back to the same we had been doing, and I think that was kind of like a tell-tale sign for us. From there we’ve kind of been trying to fight our way out of it.

“We weren’t doing the same things we were doing at the beginning of the year — executing the ball, having fun out there, sharing the ball. It’s just a lot of little stuff, and we’re slowly trying to get that back, but we have to make it more habit than anything.”

The Celtics will get their first shot to break the habit on Saturday in Game 1 against the Heat. No one on the team is dwelling in the past — focusing on a 50-win season won’t get them a title — but they have to avoid the culminating past mistakes to win the playoffs.

“I worry about what’s in front of me,” said Ray Allen. “Try to figure out every day who we need to be, what we need to do. The small little plays, they all add up.”