Head south of the border this winter and avoid the Chicago chill. Flights for this deal are available starting in the middle of October through the third week in March 2019. For college students, you'll be able to find availability over spring break. Flights are available daily, but you'll have the best luck finding tickets at this price on midweek departures.
Searching Southwest, I found seats departing MDW on Sunday, November 11 and returning from CUN on Saturday, November 17, 2018. Using the Southwest Low Fare Finder calendar, you'll be able to get a good read on seat availability per month. Peak Holiday travel windows are blacked out.
Looking for a place to stay in Cancun? Here's what's available from Hotelwatchdog for the sample dates mentioned.