Given the latest updates, it's time to review the list:
29/01/1967: apparently, Brian May was in the audience at Saville Theatre in London. That night, The Koobas, The Jimi Hendrix Experience and The Who all performed there. Freddie Mercury was maybe there too.
13/09/1967: at the time Brian May was a member of the enterteiment committe of the Imperial College in London and said in an interview that he managed to get Jimi Hendrix to play at the college. Searching in various Hendrix's concertographies, the only performance at the Imperial College was on September 13th, 1967; that night Brian attended the gig, in which also other bands performed (probably Steamhammer, Spooky Tooth and America).
It is also possible that Brian got to see several other artists playing live in that college in that years.
26/10/1968: Brian and Roger saw Pink Floyd live at Imperial College in London; also on this date, Smile opened for them (it was their first known live performance).
29/04/1969: Freddie Mercury was in the audience at David Bowie's only gig at Ealing Art College, where he was a student at the time. Freddie helped him to built the small stage where he performed that day and a few time later he opened a stall at Kensington Market with friend Roger Taylor; David Bowie visiting them one day and Freddie gave him a pair of vintage boots because he wasn't able to afford them yet.
07/06/1969: Brian, Roger, Timothy Staffell and maybe also Chris Smith attended Blind Faith's debut concert, which took place at Hyde Park in London.
09/09/1969: Brian and Roger, at the time still members of Smile, attended Ibex's concert at The Sink in Liverpool. They joined Freddie's band on stage and played guitar and drums respectivetely on the encores; the tape of that evening runs out after 30 minutes and the encores are missing.
24/09/1969 some sources claims that John Deacon (at the time a student at College of Technology in London) attended the Deep Purple concert at the Royal Albert Hall. That night, the band performed their legendary "Concerto for Group and Orchestra" with the Royal Philarmonic Orchestra; the performance was released as a live album in December 1969.
12/10/1969: Freddie went to see Led Zeppelin playing at Lyceum Ballroom in London.
30/09/1970: apparently, Freddie Mercury and Mary Austin attended Mott The Hoople's gig at The Marquee Club in London.
16/10/1970: according to a very well known source (queenlive.ca), John Deacon attended the Queen concert at College of Estate Management in London. At the time the band consisted of Freddie, Brian, Roger and Barry Mitchell.
23/03/1971: reportedly, Queen attended together the Led Zeppelin concert at The Marquee Club in London.
25/09/1971: Roger Taylor saw David Bowie's concert at Friars Club in Aylesbury.
29/01/1972: Roger Taylor and Freddie Mercury went to Friars Club in Aylesbury to see David Bowie perform (it was Ziggy Stardust at the time).
12/02/1972: Brian and Roger were in the audience at Imperial College in London during David Bowie's performance.
sometime in 1972: Brian May attended Prairie's concert at The Greyhound in Fulham.
27/03/1975: Queen attended the Led Zeppelin concert at Los Angeles Forum.
18/05/1975: Queen attended the Led Zeppelin concert at Earls Court Exhibition Arena in London.
12/06/1977: Brian attended Ian Hunter's concert at Hammersmith Odeon in London.
sometime in 1977: Brian May attended a concert by The Quick in Los Angeles, maybe at The Starwood or the Whisky A Go Go; he also offered them a support slot.
01/07/1978: apparently, Brian May and Roger Taylor attended David Bowie's third consecutive concert at Earls Court Exhibition Centre in London.
21/11/1978: Freddie, Brian and Roger (with Paul Prenter) attended Kiki Dee's concert at Exit Inn in Nashville. Queen performed the next day at Nashville Municipal Auditorium.
mid-June 1980: three members of Queen went to Flipper's Roller Boogie Palace in Los Angeles to see the Blasters perform. Later, the Blasters were hired to open for Queen during their 1980 North American tour; they were the support band from July 1st to July 14th, 1980 and August 5th, 1980.
07/12/1980: apparently, Brian attended Rod Stewart's concert at Wembley Arena in London along with Pete Townshend of The Who.
17/12/1980: apparently, Queen attended the AC/DC concert at Olympiahalle in Munich; the next day, Queen played at the same venue and AC/DC attended in the audience.
03/02/1981 (apparently): Freddie Mercury and Peter Freestone attended Giuseppe Verdi's 'Un Ballo in Maschera' at Covent Garden, London; Luciano Pavarotti and Montserrat Caballé performed together in the opera that night.
21/02/1981: Roger Taylor attended Gary Myrick and The Figures' concert at Perkins Palace in Pasadena, California. Another band called Great Buildings played that night.
May-June 1981: Roger Taylor and John Deacon attended one of Bruce Springsteen's gigs at Wembley Arena in London. Springsteen performed there on 29th and 30th May and 1st, 2nd, 4th and 5th June 1981; Roger and John attended two different concerts.
27/06/1981: during the induction of Def Leppard into the Rock And Roll Hall Of Fame in 2019, Brian May said that he went to Munich to see Ritchie Balckmore's Rainbow and he missed the opening act, Def Leppard (Queen were there to record "Hot Space"). Checking Rainbow's concertography, they performed at the Rudi-Sedlmayer-Halle on this date, the same venue where Queen performed on 10th and 11th February 1979.
14/09/1981: Roger Taylor and John Deacon, together with Tom Petersson of Cheap Trick, attended Adam & The Ants' concert at Greek Theatre in Los Angeles.
08/01/1982: Freddie was in the audience at Hog's Grunt Pub in Cricklewood during Taxi's concert; he also joined them on stage for "Jailhouse Rock".
06/06/1982: the day after the legendary Queen gig at National Bowl in Milton Keynes, Brian May attended Heart's concert at Dominion Theatre in London (Heart was one of the openicg act during the short UK tour in 1982). May also joined them on stage that night to perform some songs.
sometimes between 10/09 and 17/12/1982: according to Peter Hince's book, members of Queen (including Freddie) attended one of The Who's concerts during their Farewell Tour. The Who performed in Birmingham on September 10th and 11th, 1982, but Queen were in America at the time on their 'Rock 'N America Tour'; The Who kicked off the first leg of their North American tour on September 22nd from Landover, Maryland and continued until October 31st, when they performed in Tempe, Arizona. However, Queen performed in Japan from October 19th to November 3rd.
The Who's tour resumed on November 27th from Orlando, Florida and concluded on December 17th, 1982 in Toronto. There is no other information ascertaining the correct date Queen saw The Who in concert.
19/11/1982: Freddie Mercury attended Elton John's gig at Apollo Theatre in Manchester; he also joined Elton on stage for the encores.
10/09/1983: Brian May was in the audience at Def Leppard's concert at Los Angeles Forum. The next day, he joined them on stage at the same venue to play "Travelin' Band".
22/05/1984: Freddie Mercury attended Elton John's concert at Olympiahalle in Munich; Elton dedicated "I'm Still Standing" to him (in fact to "Melina, the poor cow"). Freddie had fractured ligaments in his leg.
16/08/1986: Brian May was in the audience during the 'Monsters Of Rock 1986' and attended Def Leppard's concert; it was the big return of drummer Rick Allen after his car accident (where he lost his arm).
29/03/1987: Freddie Mercury and Mike Moran attended Montserrat Caballé's performance at Royal Opera House in Covent Garden, London. Caballé performed "Exercises In Free Love", a new song written for her by Mercury and Moran.
18/08/1987: just one day before John Deacon's 36th birthday, he and Peter Straker attended the first Madonna's concert at Wembley Stadium in London; Madonna performed three consecutive concerts there, on August 18th, 19th and 20th,1987.
04/12/1988: Brian May and John Deacon attended The Cross' concert at Le Palais in Hammersmith, London. Later that night, they joined Roger Taylor's band on stage and performed "Dust My Broom", "I'm In Love With My Car", "Early Morning Blues" and "Whole Lotta Shakin' Goin' On".
12/12/1988: Brian May joined the band Bon Jovi on stage at Wembley Arena in London and performed The Beatles' "Get Back" and Creedence Clearwater Revival's "Travelin' Band" with special guests Elton John, Rick Allen and Lita Ford.
31/10/1989: Brian and Roger, with their respectives wives, attended The Who's concert at Royal Albert Hall in London during 'The Kids Are Alright' tour.
13/06/1992: Brian attended Guns N' Roses concert at Wembley Stadium in London; he also joined to perform "Tie Your Mother Down" and "We Will Rock You" with them. Soundgarden and Faith No More opened for Guns taht night.
11/11/2003: Brian May attended The Darkness' concert at the Astroia Theatre in London.
May 2005: Brian attended one of the reunion concerts of Cream at Royal Albert Hall in London; the trio performed there on 2nd, 3rd, 5th and 6th May 2005.
08/07/2007: Brian May, along with drummer Nick Mason, attended Genesis' concert at Twickenham Stadium in Twickenham, London.
10/12/2007: Brian attended Led Zeppelin's reunion gig at O2 Arena in London for the 'Ahmet Ertegun Tribute'.
10/10/2011: Brian May attended The Darkness' concert at 100 Club in London.
22/08/2014: Brian and Roger attended Lady Ga Ga's concert at Perth Arena in Perth. Two days later, she joined Queen + Adam Lambert on stage at the same venue to perform "Another One Bites The Dust" with them.
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