BOSTON — The Boston Celtics have made a tradition out of this time of year.

When the weather beckons people outside, with robed graduates posing around Boston Public Garden and tourists falling for the Swan Boats trap, the Celtics celebrate the end of May by hosting the Eastern Conference finals. It has become a birthright of the team’s young, bright stars who have been aged by playoff successes, and failures, and have been saddled with the kind of criticism usually reserved for superstars who fall short of expectations.

Five appearances in the conference finals in seven seasons together can equally varnish and vaporize a reputation when you’re as talented as perennial all-star Jayson Tatum, or as acclaimed as Jaylen Brown, an all-NBA selection a year ago, yet you don’t possess a championship ring to completely silence the pundits.

And so they’re at it again. These Celtics are back in another conference finals, now against the Indiana Pacers. They’re again teasing a spoiled fan base that awaits an 18th championship banner, once more proving who runs the East while breezing into the end of May, yet reminding everyone this wouldn’t be a Celtics conference finals without the crazy. It’s their tradition.

Making sure there’d be bedlam, the Celtics opened the series on their home floor Tuesday night with a 133-128 overtime win. They needed Brown to make a heroic shot late in the fourth quarter to steal Game 1.

“I think it’s always great to come out with a win,” Brown said after he scored 26 points and saved Boston by nailing a tying 3-pointer in front of the Pacers’ bench with 6.1 seconds remaining in regulation.

“Obviously, we’ve got to tighten up in some areas,” Brown continued. “I think the majority of it was just transition. We knew (the Pacers) were going to be fast. They didn’t surprise us. But it’s one thing watching it on film and then seeing it in person. They just got out of a dogfight in New York, and it just felt like they were flying by us. We got to get better. We’ve got to match their intensity if we want to win this series.”

Although the sixth-seeded Pacers – the gate crashers of the East – were coming off a seven-game rumble against the New York Knicks, they looked fresh and bouncy, rather than like a team that had just one full day to rest and prepare for top-seeded Boston. The Pacers showed they didn’t come this far for the sightseeing.

Each time Boston pushed the lead to double digits, Indiana punched right back. The Pacers didn’t need the officials’ whistles to aid their comeback. Although there were no blatant blown calls – plenty plagued the Pacers in the Knicks series – by the end of the third quarter, Indiana had been awarded just two free throws, compared with the Celtics’ 21 attempts. The imbalance didn’t harm the Pacers, and they still moved ahead 115-110 with less than two minutes left in regulation.

Watching Boston fall behind late compelled many fans inside TD Garden to leave their seats and search for the nearest exits. They had seen heartbreak before in the conference finals, including just last year when the eighth-seeded Miami Heat snatched away Boston’s home-court advantage. But things were supposed to be different this time. At least that’s what the signs say.

Different Here. That’s the theme for the 2023-24 season, introduced way back before Boston set a franchise record with 20 straight wins at home to start the year and then finished the schedule 14 games ahead of everyone else in the Eastern Conference. After the sparkling regular season, the motto has carried into the playoffs, where the Celtics needed only 10 games to get through the first and second rounds, bullying a pair of teams weakened by injuries (Miami and Cleveland). The slogan still decorates the back hallways of the arena and unfurls on a giant green flag while being waved on the court by Lucky, the human mascot dressed as a leprechaun.

Different Here. OK, but is it really? Boston might have rolled through the regular season, then glided in the opening rounds of the playoffs, but here, during this important stretch of the calendar, this team bears a striking resemblance to last year’s juggernaut that reached the Eastern Conference finals: it’s just as vulnerable.

Now playing the role of the 2023 playoff Celtics … the 2024 playoff Celtics.

There are issues with this top seed. There’s the fact that – again – the reserves might look great in their green and white warm-ups but then look unplayable for stretches out on the floor. Last year, Miami had more reliable play from its bench than the Celtics got from theirs. On Tuesday night, the Pacers’ shortened rotation – essentially consisting of two reserves in Obi Toppin and T.J. McConnell – outscored Boston’s backups 30-13. As the series moves forward, Boston Coach Joe Mazzulla may have to lean more on his starters, as he did Tuesday by playing point guard Jrue Holiday 48 minutes (11 more than he averaged during the first two rounds).

“Jrue was exceptional,” Brown said. “Shout out to Jrue. He’s the reason we’ve won this game.”

Boston can get away with wearing out Holiday, a dogged defender and former champion who had 28 points, eight assists and seven rebounds Tuesday. But there is no hiding starting center Al Horford on the floor when Pacers players are hunting him for matchups, and there’s no place to sit him on the sideline when the alternative would be giving more minutes to 7-foot-1 Luke Kornet, who would be tasked with minding versatile center Myles Turner or springy leapers such as Toppin or Isaiah Jackson.

Then there’s the matter of those vanishing double-digit leads. Boston peaked with a 13-point cushion late in the third quarter of Game 1, and considering the team’s free-throw and home-court advantage, it shouldn’t have been a hard ask to tilt the scoreboard even more. Instead, the Pacers paired their energy guys off the bench with two starters, and the Celtics squandered the lead. Then they nearly squandered the game.

“It really just felt like we had a second chance,” Tatum, who finished with 36 points and 12 rebounds, said of his feeling when overtime arrived. “The end of the third and fourth quarter, we didn’t necessarily play well enough where we may have deserved to win. We obviously made some plays at the end. J.B. gave us a second chance by hitting the shot.”

In their first real test of the postseason, the Celtics barely survived a game against a team they’re heavily favored to beat. Only after Brown submitted his latest entry for Celtics lore, and his teammates took control in overtime, did the night feel secure, even if the series does not. Then again, it wouldn’t be springtime in Boston without a little drama.