5 Seconds of Summer
5 Seconds of Summer, also known as 5SOS are an Australian pop rock band, consisting of Luke Hemmings (lead vocals, guitar), Michael Clifford (guitar, vocals), Calum Hood (bass guitar, vocals) and Ashton Irwin (drums, vocals).
Their lead singer Luke Hemmings posted a tweet that said 'seeing muse!:)", by which he most likely meant he wen't to see a Muse show at the iTunes festival that day (30th of September). You can see the tweet here: