The following blog posts mention 'september', but...

Yes, it's the Royal Decameron in Puerto Vallarta, Mexico again. They seem to be one of the few resorts that slashes their package prices to absurdly low levels every now and again, usually for last minute dates.

Sunwing just dropped the price of their September 19-25 flight to Cancun down to $335 roundtrip after tax.

Sunwing has really been pushing out the best deals lately...

Sunwing has a last minute flight to Cancun for $355 roundtrip after tax, September 12-19.

Sunwing has dropped the price of their charter flight to Puerto Vallarta over the September long weekend to $365 roundtrip after tax.

I've mentioned the Royal Decameron in Puerto Vallarta a few times now. It seems to be one of the resorts that offers significant discounts from time to time.

Allegiant Air is showing a flight from Kalispell, Montana to Las Vegas for $80 roundtrip after tax. September 13-17.

Air Transat has dropped the price of their fall flights to London Gatwick down to $629 roundtrip after tax.

A large number of Air Transat flights to Manchester in September have fallen to $584 roundtrip after tax...

Update: Looks like this is part of a price war between Delta and United for not only flights to Phoenix, but to Charlotte, North Carolina and Philadelphia as well. All 3 cities are available throughout the calendar year, including Christmas, for around $320 roundtrip after tax.