If you're flying anywhere during the holiday season, be prepared for long lines and packed planes. Some 45 million Americans will take to the skies during the 19-day period between Wednesday, Dec. 17, and Sunday, Jan. 4, up from 44 million last year, reports airline industry trade group Airlines for America. 

"Airlines are adding seats to the marketplace to accommodate increasing demand," said John Heimlich, chief economist for the group. Capacity is expected to be up by 2.4 percent. Despite that, planes are projected to be 80 percent to 90 percent full during the holiday season. 

If you haven't yet booked your seats, you may want to choose one of the lighter days to travel: Wednesday, Dec. 24, Thursday, Dec. 25, Wednesday, Dec. 31, or Thursday, Jan. 1. The day you want to avoid? Friday, Dec. 19, which is projected to be the busiest travel day of the season, when 2.5 million people are expected to fly. 

The busiest airport in the U.S. will be Hartsfield-Jackson Atlanta International Airport, followed by Los Angeles International Airport and Chicago's O'Hare International Airport.