Most fares are available in April and May, then again from September through the end-of-schedule in January. The departure days vary by route but you will have the most luck finding seats in the middle of the week, Tuesday, Wednesday, and Thursday. This is an unadvertised sale and could disappear at any moment so if you are interested in any of these deals we would recommend you book soon. AA.com allows you to hold a fare for 24-hours so if you are on the fence, for now, you could put it on hold and think it over.
As an example pictured below, we have found seats departing SEA on Thursday, September 20 and returning from MEX on Tuesday, September 25, 2018.
Routes on sale include:
If your departure city isn't listed above, then have a look over here for a list of all of today's deals to Mexico City MEX.