Following their first tours in the early 1990s, Tool has performed as a headline act in world tours and major festivals such as Lollapalooza (1997 and 2009), Coachella Valley Music and Arts Festival (1999 and 2006), Voodoo Experience (2001 and 2016), Download Festival (2006), Roskilde Festival (2001 and 2006), Big Day Out (2007 and 2011), Bonnaroo (2007), All Points West Music & Arts Festival (2009), and Epicenter (music festival) (2009). They have been joined on stage by numerous artists such as Buzz Osborne and Scott Reeder (bassist) on several occasions; Tom Morello and Zack de la Rocha during their 1991 tour; Layne Staley in Hawaii, 1993; Tricky (musician), Robert Fripp, Mike Patton, Dave Lombardo, Brann Dailor of Mastodon (band), and experimental arts duo Osseus Labyrint during their 2001–02 Lateralus tour; and Kirk Hammett, Phil Campbell (musician), Serj Tankian, and Tom Morello during their 2006–07 tour. They have covered songs by Led Zeppelin, Ted Nugent, Peach (band), Kyuss, the Dead Kennedys, and the Ramones.
Question: How many years apart were the two festival performances at Big Day Out?
Answer: 4