The best things in life are free. Especially when they’re 50,000-person dance parties.

Robin Thicke, who penned song-of-the-summer “Blurred Lines,” and indie rockers Vampire Weekend will headline the annual Virgin Mobile FreeFest music festival, slated for Sept. 21 at Merriweather Post Pavilion in Columbia, Md.

Tickets go on sale Aug. 9 through Ticketfly, but Virgin Mobile Facebook fans and past FreeFest attendees can vie for tickets the day before. The festival, which draws in about 50,000 attendees, has historically sold out in minutes.

Almost two dozen acts nabbed spots on the bill this year, including The Avett Brothers, MGMT, Icona Pop and Washed Out. The lineup swings away from last year’s rock-heavy roster, in which former White Stripes frontman Jack White headlined, bringing more electronic dance artists to the festival’s two main stages. No rap acts made the list.

The “dance forest” will make a reappearance, offering concertgoers the chance to show off their moves to artists like Ghost Beach, Pretty Lights and Madeon.

Organizers, in lieu of ticket profits, ask festival-goers to donate to The Re*Generation, a national project to address and combat youth homelessness nationwide. Tenants of the Southeast Washington homeless youth shelter created a mural with donations from previous festivals, and organizers slipped in the lineup on the artwork in an unveiling Monday at the Sasha Bruce Re*Generation House.