U.S. citizens on Thursday marked their annual Thanksgiving Day holiday — a centuries-old harvest time tradition.

The holiday is one of the busiest times for travel in the United States as millions travel by car, train, or air, to feast with family. Many also celebrated the holiday far from home and loved one — including the three astronauts on the International Space Station. The astronauts ate their turkey, sweet potatoes, and stuffing, nearly 400 kilometers above the Earth's surface.

U.S. President Barack Obama made telephone calls to some of the American troops celebrating the holiday in Iraq. It will be the troops' last Thanksgiving there since the U.S.-led invasion in 2003. Fewer than the 20,000 U.S. soldiers remain in Iraq, and they all are scheduled to be withdrawn by the end of the year.

In a radio address, the president thanked those who are taking time to do volunteer work, such as serving Thanksgiving Day meals at homeless shelters. Mr. Obama also urged U.S. citizens facing difficult economic times to believe in the nation's ability to meet its challenges. He said the nation's problems did not develop overnight and will not be solved overnight.

Thanksgiving Day has a long tradition in the United States. In 1863, President Abraham Lincoln issued a proclamation calling for the last Thursday of November to be observed as a day of thanks.