The C's saved their best for last — in the fourth quarter of Game 7 — and it's enough for them to move on to take on the Miami Heat.
The big story, of course, was Rajon Rondo, who scored nine points in the final 3:30 to propel the Celtics to victory.
It's time to head downstairs for reaction, and we'll have all of that for you shortly.
Fourth Quarter, 0:57, Celtics 81-73: Rajon Rondo has nine points since Paul Pierce fouled out, and he's the main reason the Celtics are close to putting this on ice.
He just knocked down a 3-pointer from three feet behind the line. The Garden crowd can sense it, with the Celtics up eight and under a minute to play.
Fourth Quarter, 2:47, Celtics 69-63: It's Rajon Rondo's offense now, especially with Paul Pierce on the bench, and the point guard has responded.
Rondo gets a layup to make it 73-68, and then he followed that up on the next possession with a deep jump shot. It was originally ruled a 3-pointer, but Rondo's toe was on the line.
Fourth Quarter, 4:16, Celtics 69-63: Paul Pierce is called for an offensive foul, and he fouls out with 4:16 to play.
Fourth Quarter, 5:16, Celtics 69-63: Shooters are told to just keep shooting no matter what. Ray Allen is a shooter, and in the fourth quarter, that means good things for the Celtics.
Allen just driled his second 3-pointer of the quarter and game to extend the Celtics' lead to eight, which, in a game and series like this, is a pretty big lead.
However, the Sixers cut the lead to six, and now Andre Iguodala goes to the line with a chance to cut it to four.
Fourth Quarter, 8:40, Celtics 62-55: Ladies and gentlemen, Ray Allen has hit a 3-pointer.
That hasn't been announcement-worthy in the past, but with the way Allen and the C's are shooting it from distance, it's a big deal. Unsurprisingly, the 3-pointer sent the Garden crowd into a frenzy.
Kevin Garnett has just been fantastic in the fourth quarter and going back to the third when he hit the buzzer beater. He's torching the Sixers from outside the key, and he now has a game-high 17 points.
However, it's still very much a game, especially with the pesky Sixers refusing to quit.
End of third quarter, Celtics 55-52: These Sixers, I tell ya, they're a resilient bunch.
The Celtics did all they could to pull away in the third quarter, but the Sixers did a tremendous job of weathering the storm and hanging in there. On a few occassions the Celts have looked ready to put it away, but the Sixers just hang on.
That being said, the C's got what could be a huge shot in the arm as Kevin Garnett hit a jump shot as the buzzer sounded to give the Celtics a three-point lead heading into the fourth quarter with the season on the line.
Third Quarter, 3:10, Celtics 53-45: Paul Pierce is doing all he can to take this game over right now, and he's succeeding for the most part.
The Celtics captain hit a pair of elbow jumpers in the last couple of minutes to help the C's push the lead back to eight.
With Kevin Garnett and Ray Allen on the bench, it looks like Pierce has taken it upon himself to be more assertive. So far, so good.
Third Quarter, 5:07, Celtics 49-45: It's becoming hard to watch Ray Allen play.
The Celtics sharpshooter is ice cold right now, as the Sixers almost seem to be begging him to shoot from the outside. When Allen was able to get around his defender, he missed a runner in the lane. It's just not pretty right now.
However, the Celtics continue to hold on to the lead, despite a little push from Philly. The C's extended the lead to 11 at the eight-minute mark, but the gritty Sixers have clawed back, cutting it to four.
We also just had some nastiness when Brandon Bass was shoved to the floor by Evan Turner as the two dashed after a loose ball. Turner then decided it best to step over Bass a la Allen Iverson.
Third Quarter, 12:00, Celtics 41-33: The second half of Game 7 is under way.
Halftime, Celtics 41-33: The Celtics ended the first half much the way they started it.
The C's went on a mini 7-1 run to end the first 24 minutes of Game 7, and they'll take an eight-point lead to the locker room.
A 7-1 run may not seem like much, but with the way these two teams are playing, it may end up being the difference in this one. It hasn't been a pretty, but if you're a Celtics fan, you'll take the results no doubt so far.
Kevin Garnett has really come to play tonight. He has nine points and seven rebounds, but his efforts have extended beyond the box score. Garnett's defensive intensity has been tremendous, as he's been there for help all night, while also getting in passing lanes and disrupting the Sixers offense.
Rajon Rondo has also bounced back nicely with five points, five rebounds and — you guessed it — five assists in the first half.
Brandon Bass has a game-high 10 points, while Elton Brand is the Sixers' leading scorer with eight points.
Second quarter, 2:36 Celtics 34-30: Kevin Garnett just summed up the offensive efforts of this series on one fast break gone awry.
Garnett took off down the court after a Philadelphia turnover and got out in front on transition. Rondo found him, but Garnett could not quite elevate high enough to dunk the ball, resulting in the always embarrassing missed dunk.
It was his fourth missed field goal of the night, which means four out of the five Celtics starters have now missed at least four shots. The sharp-shooting Brandon Bass has only missed three.
Second quarter, 5:17 Celtics 30-26: Rajon Rondo is doing all he can to make us all forget about his Game 6 performance.
Rondo just had one of those "Rondo possessions" to extend the lead. Rondo's hustle led to an offensive rebound, which he then took to the rim. He was fouled but still hit the shot.
This game still has a "muddy" feel — and that's putting it nicely — but you have to think the Celtics don't necessarily mind that.
Second quarter, Celtics 25-22: The Celtics have gotten some pretty solid contributions off of the bench so far, particularly from Mickael Pietrus and Keyon Dooling.
Pietrus got the crowd going in the second quarter with an emphatic dunk, and he also crashed the offensive glass for a putback on a Ray Allen miss.
Dooling, on the other hand, helped the C's wrestle the lead back with an acrobatic 3-point play going hard to the rim and finishing an acrobatic shot.
End of first quarter, 20-20: The Celtics jumped out to the early lead, but the Sixers were able to withstand that, and they've actually battled back quite nicely.
So nicely, in fact, that they have the game tied after the first quarter.
Despite falling behind 10-2 just a couple of minutes in, the Sixers dug in defensively to crawl all the way back.
First quarter, 2:40, Celtics 15-12.: You can't say enough about the job the Celtics have done defensively thus far.
They've limited the Sixers to just 12 points on 4-of-17 shooting, and that's obviously the good news.
The bad news, though, is that the Celtics haven't been much better offensively, especially since opening the game on a 10-2 run. The C's are shooting just 5-for-14 from the floor.
First quarter, 4:46, Celtics 13-6.: Neither Kevin Garnett nor Rajon Rondo had a very good Game 6, but both of them have come to play in Game 7.
Garnett looks really good on the defensive end, as he's done a great job of getting into position to help. He's also making things difficult for Philly at the rim, although Spencer Hawes is making that a little bit easier than it should be.
Rondo, on the other hand, is doing his thing at both ends of the court, already chipping in with a couple of jaw-dropping passes.
The C's haven't really cashed in on all of those chances, but if they start to do so, this might get ugly quickly.
First quarter, 8:14, Celtics 10-2.: This is exactly the start the Celtics needed.
The C's have been relentless on the defensive end, and it's leading to offensive chances, which they are certainly cashing in.
Brandon Bass has the hot hand early on, and his made jumper forces Doug Collins to take an early timeout with the Celtics jumping out to an early lead.
8:00 p.m.: If pregame warmups are any indication, it's going to be a rowdy and raucous environment for this one.
The Garden was rocking as the C's took the court, and the buzz hasn't subsided any with tip-off 10 minutes away.
7:40 p.m.: We're 30 minutes from tip (8:10 official start time), but here are just a couple of quick updates.
Keyon Dooling will play, despite his bout with the flu.
Avery Bradley had surgery on his injured shoulder, and Doc Rivers said it went well.
7:20 p.m.: The Celtics, as a team, were really bad in Game 6. It should go without saying that they'll need to be much better in this one if they want to extend the season.
If you're looking for one player who really needs to be better, it's Rajon Rondo. Much of what the Celtics want to do begins and ends with him — on both ends of the court.
He was sloppy with the ball in Game 6 on the offensive end, and on defense, he was burned all night by the Philly backcourt.
"He didn't have a game of great energy [in Game 6]," Celtics head coach Doc Rivers said before the game. "We need him to have a great defensive energy game."
They'll probably take a great offensive energy game out of Rondo, too.
7:05 p.m.: Greetings from TD Garden.
We're about an hour from tip-off beteween the C's and 76ers.
For your viewing pleasure, he's a story on the Celtics' Game 7 history in the Big Three era.
More updates in a bit.
8 a.m. ET: The Celtics didn't want it to get here, but after one of their ugliest games of the year in Game 6, the C's have one game to save their season.
Boston and the 76ers will tip off Saturday night at TD Garden in Game 7 of their back-and-forth Eastern Conference semifinal series. The winner of this game will earn a ticket to the Eastern Conference finals to take on the Miami Heat. They get to sit around and wait after dispatching the Indiana Pacers in six games.
The Celtics are looking to bounce back after a woeful Game 6 performance in which there were virtually no positives to take from the game. Boston was beat off the dribble and late getting over to help on the defensive end and committed 16 turnovers while shooting 33 percent from the floor.
The injury issue has been well documented throughout, but the Celtics have to hope that Ray Allen is feeling at least a little bit better after getting two days of rest in between games. The guard is battling an ankle injury that has slowed him for some time now. That wasn't as much of an issue once Avery Bradley was slotted into the starting lineup, but with Bradley now done for the season, much more responsibility has fallen on Allen.
That means other guys must step up, something that did not happen in Game 6. The Celtics will need much more out of Rajon Rondo after the point guard was torched by the Sixers backcourt on Wednesday night in Philadelphia.
It's all pretty simple in the end, though. If the Celtics play like they did in Game 6, their season will be over. If they play even a little bit better, they should be able to prevail in front of their home crowd and get a postseason matchup with the Heat.
We'll be providing updates throughout the day and up until tip-off at 8 p.m., so be sure to check back for plenty of coverage.