When and how did Canada eventually gain its independence from Great Britain?

Hint: Canada had a colonial history of years and the transformation of Canada from a self-governing British colony to a completely independent state was an evolutionary process that occurred in such a gradual way that independence can not be ascribed to a specific date.

Complete answer:
Though the British North America Act passed on 1st of July 1867, Canada which formed by merging three British colonies, got established as a semi-independent and self-governing dominion of the British Empire. Though Canada attained financial independence, it did not get full legal autonomy.
Through this Act, both federal and provincial governments were established and this also formed the basis for the constitution of Canada. Since it went into effect on 1st of July 1867, the day is commemorated as Canada Day (and earlier used to be known as Dominion Day).
It was in 1931, through the Statute of Westminster, dominions of Canada got formal recognition of its independence and equal status with England. Still, it took time for Canada to legally cut its ties from England. Thus after the Statute of Westminster, Canada took decades to reach towards full sovereignty. Canada adopted its own constitution and became a fully independent country in 1982. A bill was passed by the Canadian government renaming Dominion Day as Canada Day in the same year.
It was in 1867, Canada gained independence from Great Britain. Canada was self-governed but technically continued under the British rule till 1931.

Note: Canada is still a part of the British Commonwealth and Elizabeth II is Queen of Canada. But the Queen does not interfere in Canadian self-governance making her role only as essentially ceremonial. July 1 is celebrated every year as a national holiday ‘Canada Day’ which marks the independence of the country.