Southwest airlines does this just twice a year. This sale is for travel from the end of August thru just before Christmas.

The next one (in August if memory serves me) will be for travel thru about mid February.

If you are going to book an early season ski trip on Southwest, you will find that fares don't get lower than this.

I often book flights out west on Southwest during the 3-day sale in early October.  In particular for my annual late season trip in late March or early April.

Be interesting to see if fares change now that SW is not planning on overbooking flights.