Dancers celebrate on the streets of Central Square last year as the free Cambridge Dance Party coincides with an anniversary celebration for same-sex marriage in the city. (Photo: City of Cambridge)

If Facebook is to be believed, 27,000 people jammed the streets in front of City Hall for the Cambridge Dance Party last year, and it makes sense to brace for roughly that number this year – likely one reason why the four-hour party has become a five-hour party this year, starting at 7 p.m. Friday and ending at midnight.

Cambridge’s free annual dance party has taken place every year since 1996, starting with a mere 500 attendees and growing to 5,000 by 2012; doubling the next year; and jumping to 12,000 by 2014 before hitting last year’s stratospheric heights, when it coincided with an anniversary celebration for same-sex marriage in Cambridge. For a sense of how that looked, look about 33 seconds in on Hans Johnson Studio’s video tribute to the 2015 event:

https://youtu.be/DyV4lNqfCZQ

DJs spin, lights hit the crowd and the walls of City Hall after dark and, perhaps surprisingly for the restrained Cambridge, people actually dance (including to the Village People’s “YMCA” in front of an actual YMCA). Massachusetts Avenue is closed to traffic during the party between Prospect and Bigelow streets, and the No. 1 bus line ends in Central Square at 6 p.m., but is expected to resume full service by midnight.

In writer Julia Swanson’s account of a Cambridge Dance Party, “the crowd seemed exceptionally large and really, really into it. There were people of all ages enjoying everything from disco to Latin to pop music, and regardless of what was played everyone seemed to be digging it. Even if you didn’t feel much like dancing it was a pretty amazing way to spend a Friday evening in late June.”

“It’s the kind of event that reminds you that Cambridge isn’t just a city with a few big schools and lots of smart people,” Swanson wrote, “but a vibrant and fun community to be part of.”

Not everyone is aboard with the numbers drawn in past years. “Have fun moving or even breathing at this event with the amount of people that will probably attend if weather is nice,” said Facebook user Justin Anfuso, recounting the previous year’s celebration being paused repeatedly to rein in joyous crowds.

In its application to the License Commission for an entertainment license, the city said it expected a mere 10,000 people Friday.

For information on the free 7 p.m. Friday to midnight dance party at City Hall, 795 Massachusetts Ave., Central Square, call Maryellen Carvello at (617) 349-4301. This post was updated June 21, 2016, to reflect the city’s official expectations for attendance Friday.