The inaugural Boston Calling Music Festival took place this weekend at City Hall Plaza, and nearly 20,000 music fans descended on downtown Boston to sing along — sometimes in pouring rain and chilly temperatures — with 18 artists including the Shins, Fun., Bad Rabbits, and the National. Hopefully, the weather will be better for the second edition of the event, which organizers Crash Line Productions have announced will take place Labor Day weekend, Sept. 7- 8. The show will mirror this weekend’s festivities as 18 artists will take to two stages at City Hall Plaza. Headlining the second Boston Calling will be celebrated indie rockers Vampire Weekend and homegrown electro pop group Passion Pit. Other artists on the bill will include hip-hop sensation Kendrick Lamar, rowdy Jersey rockers the Gaslight Anthem, R&B vocalist Solange (who was just in town for a party at the Institute of Contemporary Art), indie folkies Okkervil River, Providence alt-country favorites Deer Tick, and local synth poppers Bearstronaut. Also playing will be Local Natives, Flosstradamus, Major Lazer, the Airborne Toxic Event, Wolfgang Gartner, Bat for Lashes, Flume, Lucius, Big Black Delta, and You Won’t. Like the first Boston Calling, the event will be co-curated by Aaron Dessner, multi-instrumentalist and songwriter with the National. The announcement for the September edition was unveiled with a video message before the National closed out the show Sunday night. Early bird tickets for Boston Calling will go on sale Friday, May 31, at 10 a.m. at www.bostoncalling.com and www.ticketmaster .com. A portion of the proceeds from Boston Calling will go to the Music Therapy Program at Boston Children’s Hospital.