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Frontier's calendar is currently extended through January 4, 2016 for all routes and all seats are available to use points on, although it may be at a higher level. If you're trying to look up flights to CUN and nothing is coming up after September 30 then its because service from your airport to/from CUN has been suspended/cancelled or if a connection is required, then there is a schedule change after that date that makes a same day connection impossible.

I certainly understand your frustration. Schedules changes tend to throw a wrench into plans regardless of the airline.

Let us know what your departure airport is and maybe we can help find an alternative.