According to a tradition starting with the induction into office of Franklin D Roosevelt, the new President is inaugurated at precisely 12 noon on January 20. This start time became law in 1933, with the ratification of the 20th Amendment to US Constitution. Prior to that, the inauguration date was March 4. This meant a lame duck period of almost 4 months between the election and the inauguration.

...the new President is inaugurated at precisely 12 noon on January 20.

President Trump’s Swearing in Ceremony and inaugural address took place on the West front of the US Capitol Building and could be viewed from along the National Mall. The ceremony was followed by a luncheon and then the parade down Pennsylvania Ave to the White house. The crowd on the National Mall was estimated by the New York times to be about a third of the size of that for Barak Obama’s 2009 inauguration. About 30.6 million Americans tuned in to watch it on TV.