BOSTON — Ever since the Boston Celtics broke up their Big Three in 2013, they’ve sat idly this time of year while LeBron James, Stephen Curry and Kawhi Leonard competed for NBA championships.

With Isaiah Thomas leading the charge, the Celtics have rejoined the fray. Now it’s time to see if they belong.

Boston beat the Washington Wizards 115-105 in Game 7 of the Eastern Conference semifinals Monday night. That earned the Celtics their toughest test yet, maybe the hardest in basketball: a series against James and the well-rested Cleveland Cavaliers. Game 1 is set for 8:30 p.m. Wednesday at TD Garden.

This is Boston’s first trip to the conference finals since 2012, the last year of an era fueled by Paul Pierce, Kevin Garnett and Ray Allen. The top-seeded Celtics are seeking an 18th championship, and first since beating the Los Angeles Lakers in the 2008 finals.

The defending champion Cavaliers won three of the four meetings between the teams in the regular season, including a dominant victory in early April with the Cavs back at full strength after early-season injuries.

But these Celtics aren’t short on belief, and their confidence has been intensified by a postseason filled with adversity. They rallied around Thomas following his sister’s sudden death in the first round, coming back from an 0-2 deficit against the Chicago Bulls. Then Kelly Olynyk and the bench came up huge in Game 7 against Washington.

But the Celtics have yet to beat the Cavs this season with Cleveland at full strength. Boston’s lone victory in the series came March 1, with Kevin Love still out after minor left knee surgery. Cleveland won the other games by a combined 35 points, including a 114-91 romp on April 5.

“We know it’s going to be tough, but at this point anything can happen and we really believe it,” Thomas said. “They didn’t give us a chance in this series. They didn’t give us a chance when we were down 0-2 in Chicago. We got the No. 1 seed, and they didn’t give us a chance. They don’t ever give us a chance and we just keep going. We don’t care about what others say.”

Thomas is averaging 25.4 points per game in the postseason, and his teammates have picked him up when he’s needed it. They offered support as he played in the first round despite his sister’s death in a car accident, and they powered through when Washington evened the second-round series at 2-2 by effectively containing Thomas’ usual scoring punch.

Boston’s mettle has been tested. Thomas had to visit the dentist after injuring at least three teeth in Game 1 against Washington, and the Celtics are only advancing after their defenders contained John Wall in Game 7.

Even more effort will be needed to compete against James and the Cavs. It starts with Jae Crowder, who is expected to be the primary defender on James, with reserve Jaylen Brown. Avery Bradley will be tasked with slowing Kyrie Irving, with Al Horford on Love and Thomas guarding sharpshooter J.R. Smith.

“I think they’re better than any of the four times we played them, including the last one when they smashed us,” Boston Coach Brad Stevens said. “So we’re going to have to play really well. … But we have a special opportunity to get a chance to compete against them.”

However this postseason ends for Boston, it looks like just the beginning for the group. But make no mistake – the Celtics want to win now.

Avery Bradley is one of the two current Celtics, along with Horford (with Atlanta), to have played in a conference finals.

Bradley was a member of the 2011-12 Celtics team that lost 4-3 to the James-led Miami Heat, who went on to claim the NBA title.

“Been here before so we have to be smart,” he said. “They’re going to bring it and try to pick us apart, and take advantage of certain matchups. … We have a lot of guys that have playoff experience. That was a lot of guys’ first year in the playoffs. We have to go out and fight.”