According to Housing Services, a shoulder season is a time of year when temperature fluctuations of more than 30 ºF within a 24-hour period are common. Although they claim this occurs primarily in late April and early May, it's really more of a year-round phenomenon.
Apparently this makes things hard on Housing's radiators, which aren't so hot to begin with. Well, actually they're too hot most of the time.
Housing's suggestion for how to deal with this? Open your window.
Email from Housing Services
April 23, 2008
As we are approaching the end of the academic school year and the outside temperatures are beginning to warm up, each of the residence halls will be undergoing a seasonal conversion in which the heating systems will be turned off and the cooling systems will be engaged. The conversion process may take up to three weeks to complete for the entire campus. For undergraduate residence halls that are equipped, air-conditioning is generally not online until May.
Throughout the month of April and often into early May, New York City weather fluctuates substantially. These periods are often referred to as shoulder seasons. Temperature fluctuations of 30°F or greater within a 24-hour period are common.
Until the season changes fully and the warmer or colder temperatures settle in, most campus buildings likely be warmer than desired. Unfortunately, this is an unavoidable, annual occurrence, and there is not much that can be done about this possible discomfort. We encourage you to open your windows when appropriate as a natural cooling means.
Thank you in advance for your patience while each of the residence halls undergoes this seasonal conversion.
It would appear that the term is actually related to travel, not weather.