List of Canadian Prime Ministers

The Prime Minister of Canada is an official who serves as the primary minister of the Crown, chair of the Cabinet, and thus head of government of Canada. Officially, the prime minister is appointed by the Governor General of Canada, but by constitutional convention, the prime minister must have the confidence of the elected House of Commons. Normally, this is the leader of the party caucus with the greatest number of seats in the house. But, if that leader lacks support of the majority, the governor general can appoint another leader who has that support or may dissolve parliament and call a new election. By constitutional convention, a prime minister holds a seat in parliament and, since the early 20th century, this has more specifically meant the House of Commons.

The office is not outlined in any of the documents that constitute the written portion of the Constitution of Canada; executive authority is formally vested in the sovereign and exercised on his or her behalf by the governor general. The prime ministership is part of Canada's constitutional convention tradition. The office was modelled after that which existed in Britain at the time. Sir John A. Macdonald was commissioned by the Viscount Monck on May 24, 1867 to form the first government of the Canadian Confederation. On July 1, 1867, the first ministry assumed office.

The date for which a prime minister begins his or her term has been determined by the date that he or she is sworn into his or her portfolio, as an oath of office as prime minister is not required. However, since 1957, the incoming prime minister has sworn an oath as prime minister. Before 1920, prime ministers' resignations were accepted immediately by the governor general and the last day of the ministries were the date he died or the date of resignation. Since 1920, the outgoing prime minister has only formally resigned when the new government is ready to be formed. The Interpretation Act of 1967 states that "where an appointment is made effective or terminates on a specified day, that appointment is considered to be effective or to terminate after the end of the previous day." Thus, although the outgoing prime minister formally resigns only hours before the incoming ministry swears their oaths, both during the day, the ministries are effectively changed at midnight the night before. Some sources, including the Parliament of Canada, apply this convention as far back as 1917.

...
Wikipedia