who is believed by some people to have been the 12th President
of the United States
for exactly one day, Sunday, March 4, 1849
Atchison was president pro tempore of the Senate at the time. James K. Polk's term ended that day, and president-elect Zachary Taylor refused on religious grounds to be inaugurated on a Sunday.
He slept through most of his tenure in office, doing nothing at all of consequence -- starting no war, creating no new tax, and of course abstaining from photo ops. For this reason, he is sometimes lauded as not only President for a Day, but the best President so far.