Thanksgiving is a national holiday celebrated on various dates in October and November in the United States, Canada, Saint Lucia, Liberia, and unofficially in countries like Brazil and Germany. It is also observed in the Australian territory of Norfolk Island. It began as a day of giving thanks for the blessings of the harvest and of the preceding year. Various similarly named harvest festival holidays occur throughout the world during autumn. Although Thanksgiving has historical roots in religious and cultural traditions, it has long been celebrated as a secular holiday as well.

Prayers of thanks and special thanksgiving ceremonies are common among most religions after harvests and at other times of the year. The Thanksgiving holiday's history in North America is rooted in English traditions dating from the Protestant Reformation. It also has aspects of a harvest festival, even though the harvest in New England occurs well before the late-November date on which the modern Thanksgiving holiday is celebrated.

In the English tradition, days of thanksgiving and special thanksgiving religious services became important during the English Reformation in the reign of Henry VIII. Before 1536 there were 95 Church holidays, plus every Sunday, when people were required to attend church and forego work. Though the 1536 reforms in the Church of England reduced the number of holidays in the liturgical calendar to 27, the Puritan party in the Anglican Church wished to eliminate all Church holidays apart from the weekly Lord's Day, including the evangelical feasts of Christmas and Easter (cf. Puritan Sabbatarianism). The holidays were to be replaced by specially called Days of Fasting and Days of Thanksgiving, in response to events that the Puritans viewed as acts of special providence. Unexpected disasters or threats of judgement from on high called for Days of Fasting.

Special blessings, viewed as coming from God, called for Days of Thanksgiving, which were observed through Christian church services and other gatherings. For example, Days of Thanksgiving were called following the victory over the Spanish Armada in 1588 and following the deliverance of Queen Anne in 1605. An unusual annual Day of Thanksgiving began in 1606 following the failure of the Gunpowder Plot in 1605 and developed into Guy Fawkes Day on November 5. Days of Fasting were called on account of plagues in 1604 and 1622, drought in 1611, and floods in 1613. Annual Thanksgiving prayers were dictated by the charter of English settlers upon their safe landing in America in 1619 at Berkeley Hundred in Virginia.

According to some historians, the first celebration of Thanksgiving in North America occurred during the 1578 voyage of Martin Frobisher from England in search of the Northwest Passage. Other researchers, however, state that "there is no compelling narrative of the origins of the Canadian Thanksgiving day."

Antecedents for Canadian Thanksgiving are also sometimes traced to the French settlers who came to New France in the 17th century, who celebrated their successful harvests. The French settlers in the area typically had feasts at the end of the harvest season. They continued throughout the winter season, even sharing food with the indigenous peoples of the area.

As settlers arrived in Nova Scotia from New England after 1700, late autumn Thanksgiving celebrations became commonplace. New immigrants into the country—such as the Irish, Scottish, and Germans—also added their own traditions to the harvest celebrations. Most of the U.S. aspects of Thanksgiving (such as the turkey) were incorporated when United Empire Loyalists began to flee from the United States during and after the American Revolution and settled in Canada.

In 1859, the government of the provinces of Canada declared a Thanksgiving Day in which "all Canadians [were asked] to spend the holiday in 'public and solemn' recognition of God's mercies." On 9 October 1879, Canada's Governor General, the Marquis of Lorne, declared November 6 as "a day of General Thanksgiving to Almighty God for the bountiful harvest with which Canada has been blessed." The Canadian Parliament on 31 January 1957 applied the same language in its proclamation for the modern holiday: "A Day of General Thanksgiving to Almighty God for the bountiful harvest with which Canada has been blessed—to be observed on the second Monday in October."