Thanksgiving has been officially celebrated as an annual holiday in Canada since November 6, 1879, when parliament passed a law designating a national day of thanksgiving. The date, however, was not fixed and moved earlier and later in the year, though it was commonly the third Monday in October.
On January 31, 1957, the Governor General of Canada Vincent Massey issued a proclamation stating: “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.” (Wikipedia)