The Conclave meets in Oldtown to discuss the changing weather patterns. They agree that summer has ended and that autumn has arrived. As is traditional, they send this news to all corners of Westeros with special white ravens. Grand Maester Pycelle relays this news to the small council, prompting them to start tallying the supplies and stores the people of the Seven Kingdoms have put aside for the coming winter.
In the books
In the A Song of Ice and Fire novels, the Conclave is made up of the ruling archmaesters of the Citadel. In addition to discussing the change of seasons the Conclave also elects the new Grand Maester after the passing of the old, decides on policies, and attends to the administration of the Citadel as well as the training of new recruits. The Conclave also appoints the Seneschal, the maester who has to run the bureaucracy of the Citadel on a day-to-day basis, which is considered drudgery.
Much like the professors in a university faculty, each of the archmaesters is the foremost expert in a specific field, such as the archmaester of medicine or the archmaester of economics, etc. This is in contrast with the ruling council of the Faith of the Seven, the Most Devout, whose members have no official distinction between themselves. While each archmaester is thus not interchangeable, in theory each archmaester is equal, each casting one vote when electing a new Grand Maester. In practice, of course, archmaesters of certain areas of study are considered more prestigious than others. For example, the archmaesters of medicine or economics are held in high esteem, while archmaesters of obscure topics such as magic tend to be ostracized.