surprise concert; guerrilla live
A live performance, typically by a musician or band, announced with little or no advance notice, often in an unexpected location. The term is wasei-eigo from English 'guerrilla' and 'live'.
wasei-eigo (Japanese-coined English), from guerrilla live
The popular band held a surprise concert in Shibuya.
Information about the guerrilla live was suddenly announced on social media.