Miranda Lambert surprised boyfriend Anderson East onstage in Buffalo, N.Y., on Sunday night (July 17), singing a slow, dark version of "Happy Birthday" with the fans.

East was opening Chris Stapleton's set at Darien Lakes Amphitheater, and toward the end of the show, Lambert walked onstage. The Boot reports that Lambert had been watching much of the show from the sound booth. Monday (July 17) is East's 30th birthday.

See Pics of Miranda and Anderson Together at the ACMs

This wasn't Lambert's only onstage appearance. She joined Stapleton set, too, to sing "Fire Way." East would return with black balloons that spelled out "30" for a brief cameo during "Tennessee Whiskey." More black balloons were tossed into the crowd by Stapleton's crew, which elicited a chuckle from Stapleton.

This particular trio of Lambert, East and Stapleton have made headlines before. In April 2016 they sang "My Girl" together in Los Angeles. Lambert has also surprised East onstage to sing Mariah Carey's "You'll Always Be My Baby."

Lambert continues to enjoy some rest between tour dates. After a few shows in mid-June (including a stop at Taste of Country Music Festival) Lambert is on break until July 21. East has been a regular opener for Stapleton, but he's only one of many on the All-American Road Show Tour. Brothers Osborne, Lucie Silvas, Brent Cobb and Margo Price are also opening select shows. On Sunday night, the bearded singer debuted a new song called "Trying to Untangle My Mind."

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