Every four years on 20th January, a new U.S. President is sworn in, on an inauguration day which is set by law. Election Day is always the Tuesday after the first Monday in November and Inauguration Day is always 20th January.
Presidential Inauguration Day was on 4th March from 1793-1933. Back then, it was a bit different though - George Washington's speech was just 135 words long! Modest celebrations like that are out of the question now. In 1809 an inaugural ball was introduced, and since then, the event has grown and grown, with days of parades, prayers and presentations.
On this day in 2009, Barack Obama made history by being the first president to fail to say the oath correctly, so he had to repeat it the following day, and wasn't officially sworn in as the 44th President until 21 January.