Monday is when Southwest will mark the 30th anniversary of its entry into the Chicago air travel market.

If you happen to be passing through Chicago's Midway Airport on Tuesday, get ready to celebrate. Southwest Airlines, its employees at the airport and passengers traveling to or from Midway sure will be.

Tuesday is when Southwest (NYSE: LUV) will mark the 30th anniversary of the low-fare behemoth's entry into the Chicago air travel market — a move that without question has had a significant impact on aviation history in the city.

Of course, a lot has happened at Southwest (and at Midway for that matter) in the intervening 30 years. And most of it has to do with growth.

When Southwest started flying out of Midway in 1985, the airline's route system from the airport amounted to just seven flights daily between Midway and Lambert-St. Louis International Airport.

But an impressive thing has occurred over time.

Southwest has spread its wings in a big way at a now hugely-remodeled and expanded Midway. Today Southwest operates 250 daily nonstop flights from Midway to 70 domestic destinations and several more near-in international markets, thanks to the carrier's merger with AirTran, which was completed in late 2014.

Midway Airport also over time has become Southwest's largest hub.