Jalsa Salana (Annual Convention) was originally started in 189? by Mirza Ghulam Ahmad as an annual gathering of his devotees, to take place every year in Qadian, on the 26th, 27th, and 28th days of December.
Since then, the Qadian Jalsa has been suspended a couple of times, and now the dates appear to have been changed to May/June, since 2009.
After the partition of the Indian sub-continent in 1947, the Jalsa started to take place in Lahore, and then in Rabwah, since re-named Chenab Nagar. During the administration of Pakistani President Zia-ul-Haq, municipal permission was usually not granted, on the basis that it would cause public disturbance. Since then, the Pakistan Jalsa has not been regular.
Other countries hold this convention on a regular basis.
The convention one of the high points of the Ahmadiyya calendar, and attendance is considered a religious as well as a social must.