My sister works in the chicago Sw office and gave me a few items to pass on through the years, When using their web site click on travel tools and it will tell you when the open schedule dates are and when the next segment will open. She says buying on the first few days of the open schedule will be the cheapest offer until cancellations roll in and they need to fill seats. Also for the 1st 25% of the flight seats will be your lowest fares, between 26% and 66% of seats sold will be the next level of prices and than from 67%-80% will be the 3rd tier and after 80% they are at the profit mark and thats the highest rates you will see. The ding specials are offered if they drop below a line with a certain number of days remaining to the flight, like if someone cancels and they will offer a limited number of seats to get back over that number, or if its within the last 72 hours and there are still seats unpurchased, but SW is known for overbooking flights, and las vegas and orlando being 2 of their more popular destinations they are often over booked and that is where they see some angry passengers. Passenger 1 could book there flight 120 days in advance days for a family reunion and passnger 2 could book 48 hours in advance just out of boredom but if passenger 2 does the online check in first and passenger 1 waits til they get to the airport or does online check in to late they can be bumped from a flight that everyone else has checked in prior to them. So when it comes to buying tickets, checking in or seating onboard the plane everything truly is first come first serve.