Before the 20th Amendment in 1933, all presidents were sworn in on March 4.
It was introduced to shorten the “lame duck” period between elections and the start of new administrations.
Given that followers of the sovereign citizen movement reject all constitutional amendments passed after the 14th amendment, they do not view this date change as legitimate.
QAnon followers, who failed to see Trump inaugurated in January, have recycled the argument and reinvented the next legitimate inauguration date.
They say that on March 4, 2021, Trump will succeed the last legitimate president, Grant, to become the 19th president.