Weekly tango master classes with Mónica Parra & Demian García, followed by free milonga at Si Sos Brujo.
A guess
A late night: movement, a little warm-up shyness and then loosening up. Probably people who like to dance, beginners welcome to join in. In Buenos Aires people dine late and go out after 23:00.
A guess from the kind of event, the hour and the city, not from the organizer.