The inauguration day in the United States marks the day when a new president takes office. Because of how the government functions, there needs to be a transition between the old and the new administration. That’s why there is a transition period between the end of the elections.
The inauguration day is January 20th of the year following the election year. In the case of 2021, the inauguration day is January 20, 2021. This is the day current president-elect Joe Biden will sworn in and become the president of the United States for at least four consecutive years.
After a long and heated election, the Congress certified Joe Biden’s victory after the riots at the US Capitol. In a matter of days, there will be a transition from the Trump administration to Biden’s administration. You can read what Joe Biden promises to change and improve during his presidency from here.