In the United States, state and federal elections are always held on Tuesdays in compliance with an 1845 law passed by Congress. The law intended to align the timing of elections in all states for national elections, and chose Tuesdays because of transportation concerns, according to an article at NPR.org.
In 1845, the primary mode of transportation was the horse and buggy, and voters needed sufficient time to travel to polling places. Because Sunday was a day of worship and Wednesday was usually a market day, Tuesday became the standard election day. This allowed voters to use Monday as a travel day to reach their assigned polling places in time to vote, according to NPR.org.
Congress passed an additional law in 1875 that assigned Tuesday as voting day for the U.S. House of Representatives, and another in 1914 specifying Tuesday for U.S. Senate elections, says WhyTuesday.org. WhyTuesday is a nonprofit voting rights group formed in 2005 to advocate for changes in the voting system. The group says that because the original reasons for specifying Tuesday as voting day are no longer a concern, changes in the law to allow weekend voting would expand voter participation. They cite studies showing that Tuesday is now a work day for many people.