THE TROUBLE with being a magazine editor is there's always someone like Peter Stevens to point out your mistakes. Several Canadian magazines last year informed their readers that Canada's Thanksgiving Day originated as an expression of English explorer Martin Frobisher's relief when he landed on Baffin Island in 1578. Not so, says Stevens, a York University PhD history candidate who's written a paper on the subject. Canada's Thanksgiving really didn't get started until 1859, he says. That was when Protestant church leaders in Ontario successfully lobbied the federal government to create our first national Thanksgiving. "They wanted to make Thanksgiving a devotedly religious, Protestant, Anglo-Saxon event. The government's declaration asked all Canadians to spend the holiday in 'public and solemn' recognition of God's mercies." But the clergy were soon disappointed. By the 1870s, US holiday traditions, such as family gatherings for turkey dinner and stories of the pilgrims, took hold in Canada, creating both commercial opportunities for businesses and a way for Catholics to celebrate the day as a non-religious event. Sad to say, Thanksgiving, like all national holidays, is an invented tradition. "These inventions seek sanction from the authority of history, and therein lies both their power and their pretence," says Stevens. "We tend to assume they just happened. They didn't. There were people behind them with agendas."