The first Tuesday after the first Monday in November every other year. A more common way to say this is the first Tuesday in November but the it is sated the prior way in the constitution.
Run-Off
Those are commonly known as mid-term elections.
The main Presidential and Congressional elections are held in November. Primary elections and caucuses are held in prior months.
It's the day when elections are held (and the name The Day When Elections Are Held was too long).
There is always a congressional election held at the same time as the presidential election. However, since Congressmen serve for only a two-year term and the president serves for four years, there is a another Congressional election in the middle of the president's term. ( I am talking about the lower house of Congress. The senate elects one-third of its members every two years, so one-third of the senators are also elected at the same time as the President. )
The first Tuesday following the first Monday of November is when Congressional elections are held.
A special election, or a run-off would be the term for such an election. However, a majority is not required in congressional elections. The candidate with the most votes wins. (You may be thinking of congressional primaries used to determine a party's nominee. In this case, if there are several candidates receiving votes, a run-off might be held between the top two to determine the strongest candidate for the fall election. )
Sometimes, I think it depends on where you live, and what day your local election is held on.
Congressional elections, like all other elections in the US, except for certain very local elections, take place on the first Tuesday in November. Congressional election are held in even numbered years.
"Midterm elections" in the United States are the Congressional elections held halfway through a Presidential term, in November of even-numbered years that are not divisible by 4. For example, the November 2010 election was a midterm election, held throughout the United States.
Most elections are held on a Tuesday.
Election Day is the Tuesday following the first Monday in November, so that date will be between November 2nd and November 8th. Congressional elections are held every two years (the term of election for the House) and Presidential elections every four years.