Long Island’s biggest music festival is getting bigger.

The Great South Bay Music Festival will expand from three days to four at Patchogue’s Shorefront Park this July. While the entire lineup hasn’t been announced yet, opening night will feature co-headliners Colbie Caillat and Christina Perri on July 16.

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Caillat’s hit “Bubbly” is still one of the top-selling digital singles ever and last year’s “Gypsy Heart” album generated another hit with her Babyface collaboration “Try.” Perri is still promoting her “Head or Heart” album, featuring the hit “Human.”

Caillat and Perri have teamed up for the co-headlining tour “Girls Night Out, Boys Can Come Too,” which launches at the Mohegan Sun Arena on July 10. The singers will also stop at JBL Live at Pier 97 in Manhattan on July 14.

Tickets for the Great South Bay Music Festival show go on sale at 10 a.m. Friday through greatsouthbaymusicfestival.com. Tickets for the Pier 97 show also go on sale Friday, through Ticketmaster.