UPDATE: There are still good fares to Cancun as of Jan 16, but most now are not one-day advance purchase.
Not all fares are created equal, to paraphrase Animal Farm. A low fare with a one-day advance purchase is better than one with a 21-day. Anyway, American and United are having an unadvertised sale--they're the best kind--to Cancun from several cities. Fares are crazy low, even with a longer advance purchase, but these are pack and go specials. Generally, travel is available through April 1, but many dates are sold out so you MUST be flexible in your travel dates. Delta also has some super low fares.
You can see some of the deals here.