Tickets at this price are available starting in early January with returns available through May 1, 2019 – unfortunately just before Cinco de Mayo. Departures are valid daily except on Fridays, but you'll have the best luck finding seats with midweek flights.
Searching AA.com, I found seats departing DFW on Tuesday, February 12 and returning from CUN on Wednesday, February 20, 2019. January and February have some of the best availability left at this price.
Have another tropical destination in mind? Make sure to take a look at a full rundown of deals to sandy and sunny Beach Destinations here.
Want a room with an ocean view? Here's what's available for the mentioned dates on Hotelwatchdog.