A crowd of 41,418 — the largest since David Beckham visited 10 years ago — began arriving at the rusty venue on East Capitol Street, soaking in both a warm autumn sun and the nostalgia of a team and league that will relocate to modern digs next year.

At halftime, when United’s 2-1 defeat to the New York Red Bulls became an official match, the last numbered banner used in a season-long countdown of home dates was peeled away in the upper deck, exposing a fresh message: “Audi Field 2018.”

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With emotions and memories flowing, with former players and coaches bearing witness and an original league arch-rival providing opposition, United completed a forgettable season in an unforgettable two-plus decades at the 56-year-old relic.

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Three miles away at Buzzard Point, 20,000-capacity Audi Field is rising two blocks from Nationals Park. United plans to open the gates in June following a heavy stretch of away matches and perhaps a few “home” games at other locations in the region.

“It’s been an emotional day,” said United Coach Ben Olsen, who played the entire 50-minute alumni match — and “scored” by throwing the ball into the net — before changing into a suit and tie.

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“There’s an emotional level to this, but there’s also an excitement obviously with Audi Field and also just to get this season over with and turn the page.”

Before turning the page, United celebrated its past, one that included four league titles and several other trophies. Dozens of former players participated in weekend festivities. On the 20th anniversary of the 1997 MLS Cup championship, Arena, John Harkes and others paraded with the gleaming hardware.

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It was a particularly touching moment for Harkes, whose son, Ian, is a rookie midfielder.

“You look back,” John Harkes said, “and you think about how lucky we were to have so many great moments.”

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“It was a great day to celebrate RFK and see all the players back, guys I grew up watching,” said Ian, a Gonzaga High graduate from Fairfax. “It’s the result we didn’t want, but just give the fans one last time before we move into a new era.”

With a third consecutive defeat, United (9-20-5) finished last in the Eastern Conference and tied with the Los Angeles Galaxy (8-18-8) for the fewest points (32) in the 22-team league. (With a superior victory total, D.C. finished ahead of the Galaxy.)

Locked into the last playoff slot and awaiting a first-round match midweek, the Red Bulls (14-12-8) rested many regulars and brought only five reserves instead of the usual seven.

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Energized by the huge crowd — 2 1/2 times above average — United was on its front foot almost the entire first half. Typical of the dour season, however, numerous opportunities were squandered.

Alas, a breakthrough came just before intermission as Luciano Acosta crossed to Paul Arriola for an eight-yard volley and his first goal since joining the club in August.

The upper deck bobbed like it did in the golden days of RFK sporting events (Senators, Redskins, Nationals, United, World Cup, etc.). Contained by the wavy roof, the sound ricocheted around the cement structure for what seemed like several minutes.

With the goal, United avoided setting the MLS record for most scoreless performances in a season; it claimed a share of the mark (with the 2010 United squad) with 17 shutouts in 34 outings.

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After United missed additional chances, the Red Bulls drew even in the 68th minute on sweet combination work between Michael Amir Murillo and Muhamed Keita. Murillo lifted the ball over goalkeeper Steve Clark.

Two minutes later, Acosta was red-carded for a moment of petulance. He missed a glaring opportunity, took offense to Alex Muyl’s bump, then kicked a New York player. The suspension will carry over to the 2018 opener.

New York went ahead in the 75th minute. Muyl, a former Georgetown standout, crossed from the right side. Clark failed to corral the cross and Nick DeLeon failed to contain Gonzalo Veron, who scored from close range.

Clark prevented further damage in the 84th, stopping Veron’s penalty kick following defender Jalen Robinson’s handball.

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Referee David Gantar allowed 7 1/2 minutes of stoppage time for injuries and video review and was followed off the field by an irate Olsen, ever the feisty competitor, even in a meaningless match.

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Afterward, current players were recognized and the attending alumni were introduced. They were presented game day scarves by Kathleen Kennedy Townsend, the eldest of Robert and Ethel Kennedy’s 11 children.

Then there were fireworks and a final walk to the locker room.

Said United General Manager Dave Kasper: “A pretty emotional day, just to take that all in and say goodbye one last time to this building.”

Notes: Goalkeeper Bill Hamid, an impending free agent who is preparing to join Danish club Midtjylland in January, watched from the mezzanine level. … The Red Bulls won the Atlantic Cup, the annual trophy awarded to the winner of the season series. They won twice and tied once.