Passage: After the Brothers in Arms tour ended Mark Knopfler took a break from Dire Straits and during 1987 he concentrated on solo projects and film soundtracks. Dire Straits regrouped in 1988 for the Nelson Mandela 70th Birthday Tribute concert staged on 11 June 1988 at Wembley Stadium, in which they were the headline act. They were joined for their set by Eric Clapton who performed his hit "Wonderful Tonight" with the group and played rhythm guitar on the other songs performed by the band, while guitarist Jack Sonni was absent. Soon afterwards, Williams left the band.

Question: What year was Nelson Mandela's 70th birthday celebration?
1988