For the most part, congressional elections take place on Election Day, which is the first Tuesday after the first Monday in November, every even-numbered year. Special elections may be held on other dates, and each state can set its own primary elections to determine the final candidates.
Every two years, the entire House of Representatives comes up for election, since each member serves a two-year term. Senators serve a six-year term, so each electoral cycle approximately one-third of the Senate is up for re-election. This proportion may vary due to early retirements, deaths or other factors that might leave a seat vacant before the six-year cycle is concluded.