On December 11, 1931, Canada acquired full legal autonomy from Britain. It was the date that the Statute of Westminster was passed that established the Crown Dominion of Canada as independent of the British Parliament. Despite being granted the right to govern itself way back in 1867, Canada did not enjoy full legal autonomy until December 1931.
In 1982, Canada took the last step to become a totally independent country that could legally amend its constitution instead of depending on Britain in any way. It was Canadian Prime Minister Trudeau who helped bring in the final change.