Scheduling Presidential Inaugurations

The Twentieth Amendment of the U.S. Constitution mandates specific scheduling of presidential and Congressional terms.

Prior to the passage of the Twentieth Amendment in the early 1930s, Inauguration Day was always March 4, the anniversary of the Constitution first taking affect in 1789. The Twentieth Amendment changed the date of Inauguration Day to January 20, unless that falls on a Sunday, in which case the date is moved to January 21.

https://historyinpieces.com/research/presidential-inauguration-dates