The ASA holds our meeting in October for a few reasons. William Thomas Green Morton, an American dentist, first publicly demonstrated the use of inhaled ether as a surgical anesthetic on October 16, 1846 at the Ether Dome of the Massachusetts General Hospital. When ASA was founded, it made sense to honor the creation of anesthesia by holding the world's largest anesthesia educational meeting around that date.
In addition, we choose dates for the meetings based on the availability of reasonable rates for using the conference site, hotels, etc. We also try to schedule our meetings for a period of time when most academics around the world are available. This is usually early-October for European sites and mid-October for North American sites.