The nature of democracy is that elected officials are accountable to the people, and they must return to the voters at prescribed intervals to seek their mandate to continue in office. For that reason most democratic constitutions provide that elections are held at fixed regular intervals. In the United States, elections for public offices are typically held between every two and six years in most states and at the federal level, with exceptions for elected judicial positions that may have longer terms of office. There is a variety of schedules, for example presidents: the President of Ireland is elected every seven years, the President of Russia and the President of Finland every six years, the President of France every five years, President of the United States every four years.
Pre-determined or fixed election dates have the advantage of fairness and predictability. However, they tend to greatly lengthen campaigns, and make dissolving the legislature (parliamentary system) more problematic if the date should happen to fall at time when dissolution is inconvenient (e.g. when war breaks out). Other states (e.g., the United Kingdom) only set maximum time in office, and the executive decides exactly when within that limit it will actually go to the polls. In practice, this means the government remains in power for close to its full term, and choose an election date it calculates to be in its best interests (unless something special happens, such as a motion of no-confidence). This calculation depends on a number of variables, such as its performance in opinion polls and the size of its majority.
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