Pearl Jam's highly anticipated 20th anniversary music festival is well under way at Wisconsin's Alpine Valley Music Theater. Day one of the two-day festival took place on Saturday (Sept. 3), and continues tonight (Sept. 4).

Fans who braved the wet weather for Saturday's event had many special surprises to enjoy throughout the day, including Soundgarden's Chris Cornell, who joined the band for three Temple of the Dog songs -- including 'Hunger Strike' (see video below) -- and one Mother Love Bone cut. Mudhoney also took the stage with Pearl Jam to cover MC5's 'Kick Out the Jams.'

Throughout the evening, many other collaborations would take place, including Liam Finn and Swell Season's Glen Hansard, who joined Temple of the Do /Pearl Jam/Soundgarden on 'Reach Down.'

On the festival's second stage, Pearl Jam bassist Jeff Ament, drummer Matt Cameron and guitarist Mike McCready sat in on Joseph Arthur's set, giving fans a nod to a forthcoming album the four of them have made.

Pearl Jam frontman Eddie Vedder joined the Strokes during their 45-minute set to duet with them on their 2005 hit 'Juicebox.' And Strokes singer Julian Casablancas returned the favor by trading vocals with Vedder on 'Not for You.'

Queens of the Stone Age frontman Josh Homme also made an appearance during PJ's set for 'In the Moonlight.'

Pearl Jam closed out the evening with an array of their hits over the past 20 years, including 'Release' and 'Better Man.' "It's hard to imagine," Vedder said from the stage [quotes via Billboard]. "When you're a kid, you think that music is the most powerful thing in the universe [but] adults around you being practical say that [success in a band] ain't gonna happen. Get a nice construction job. We didn't listen. I want to thank the kid I was for hanging onto his passion."

Later this month, the band's new documentary, 'Pearl Jam Twenty,' will debut at the Toronto International Film Festival on Sept. 10. The film's soundtrack will hit stores Sept. 20.

Pearl Jam will continue to keep things moving at full speed ahead as they launch their new tour this fall throughout Canada and South America. Click here for a complete list of dates and appearances where Pearl Jam will be during the remainder of the year.

Watch Chris Cornell and Pearl Jam Perform the Temple of the Dog Hit 'Hunger Strike'