GanjaGoblin said
Why does Canada celebrate Thanksgiving?  

It is a celebration of the harvest. It has occurred in many cultures all over the world. 

Americans tend to think Thanksgiving is an American thing but it is prevalent in cultures that relied on the fall harvest to sustain them through winter months.

The indigenous people in western hemisphere have long held communal feasts in celebration of the fall harvest that predates the arrival of European settlers.

While Thanksgiving as an official Canadian holiday is relatively new (parliament declared it in 1957), the non-indigenous Canadian Thanksgiving celebrations have been happening for centuries. The first being in 1578, many years before the U.S. first Thanksgiving of 1621 at Plymouth Rock.