Inaugural Tickets: Past Practices and Considerations for Congress [November 17, 2020] [open pdf - 650KB]
From the Document: "Since the 20th Amendment was ratified, the oath of office for President of the United States has been administered every four years on January 20. The next presidential inauguration is scheduled to take place on January 20, 2021."
CRS In Focus, IF11688
Congressional Research Service: https://crsreports.congress.gov/