The first night of the 2015 Almost Acoustic Christmas took place on December 12.
The concert was the 2015 edition of KROQ’s Almost Acoustic Christmas, a major alternative-rock radio event held at The Forum in Inglewood, California. That year’s festival ran across two nights, December 12 and 13, with the first night scheduled for Saturday, December 12.
Night one featured acts including Disclosure in a DJ set, Weezer, Twenty One Pilots, Cage the Elephant, Bastille, Silversun Pickups, AWOLNATION, X Ambassadors, Foals, Halsey, and The Struts. The second night had a different lineup, led by Fall Out Boy and including Of Monsters and Men, Chris Cornell, Cold War Kids, Panic! at the Disco, and The 1975.
A frequent source of confusion is mixing the event date with its announcement, ticket-sale date, or the second concert night. Because the question asks for the first night of the 2015 two-night event, December 12 is the single canonical answer.