Stolen from a newspaper... San Jose Examiner.
It's actually kind of complicated but the short answer is because it used to take at least an overnight trip for voters to get from their homes to polling locations. Our nations leaders at the time didn't want people to have to travel on holy resting days. So they figured voters could load up their horse and wagons on Monday, make it to vote by Tuesday, then be back on the road and home on Wednesday. Makes standing in a long line seem like nothing, huh?
Here are some other fun facts about elections and our presidents:
President Andrew Johnson never went to school. He was totally self-taught.
President William Howard Taft became the first President to walk into the Oval Office (in 1909).
President John Adams was the first president to live in the White House.
President Theodore Roosevelt was the first president to ride in a car while president.
President Millard Fillmore's wife Abigail started the White House library collection.
President Martin Van Buren gave us the word okay. It was an abbreviation for the name of his New York home "Old Kinderhook." A club called the "O.K. Club" formed and soon "OK" came to mean "all correct."