 Kyiv City officials have declared today an official day of mourning. As Americans headed home from July 4th festivities, the summer travel boom reached a new record high Sunday. The TSA says for the first time, more than 3 million people passed through its airport security checkpoints in a single day. Experts say cheaper airfares and a stronger U.S. dollar are motivating Americans to fly and helping their vacation money go further. The results? Nine of the 10 busiest air travel days in the TSA's history have occurred since Memorial Day weekend.