The iTunes Festival is already on full swing and it's about to get bigger than ever as Sir Elton John joins the mix of artists tomorrow.

Apple's own music fest has grown into one of the biggest events in the industry each year. Held at The Roundhouse Art Centre in London, this year's month-long concert series is jam packed with the hottest artists of this generation - Lady Gaga, Justin Timberlake, Katy Perry, Paramore and more. On Sept. 12, the list will even be bigger with the inclusion of music royalty, Elton John.

The icon will be performing live in an audience of thousands at the theatre and millions watching online at home. He is expected to sing his greatest hits at the Roundhouse, where he has been a constant guest since the conversion of the railway engine shed to a performing arts venue.

"I think it's great that Apple makes these concerts accessible to everyone," the 66-year-old singer-songwriter told the Daily Mail online in July. "For me, it's a real thrill to play for young audiences - they've got so much energy and they all seem to know my songs!"

According to the report, Mr. John will be debuting his first album in seven years at the festival. This will also be added to the list of artists who debuted their new albums at the event, including Lady Gaga who performed her new album Artpop in full at the opening day of the fest, noted Apple CEO Tim Cook in his Keynote speech yesterday.

The iTunes Festival will run for 30 days, starting last Sept. 1 and will run until the 30th. Each night a new set of performers will grace the stage in London. Tickets to the event are free, and according to Cook, over 20 million people applied to get free tickets to the shows. The entire festival is also live streamed online for free on the Apple iPhone, iPad, Apple TV, Mac computers and on personal computers with the iTunes app.

To find out the complete schedule of the festival or to wtach the live streaming of the performances, visit the iTunes Festival site here.