Flight
The closest big airport to Longmont is Denver International Airport (DIA). It is a United hub but you can also very easily get flights through most any major airline. It is about a 40 minute drive from the airport to Longmont.
Travel Note
If you plan on visiting Denver while you are in town we would recommend getting a rental car. If you plan on just coming to Longmont and then back to the airport Lyft/Uber is a possibility but can range anywhere from $50-$100 one way. While in Longmont, the hotel is close to the venue but not within walking distance.
Travel Note