Why Thanksgiving is not the same date in USA and Canada ? Do those 2 holydays got the same meaning ?

the reason for these is The history of Thanksgiving in Canada goes back to an English explorer, Martin Frobisher, who had been futilely attempting to find a northern passage to the Orient. He did, however, establish a settlement in Canada. In the year 1578, Frobisher held a formal ceremony in what is now the province of Newfoundland and Labrador, to give thanks for surviving the long journey. This event is widely considered to be the first Canadian Thanksgiving, and the first Thanksgiving celebrated by Europeans in North America. More settlers arrived and continued the ceremonial tradition initiated by Frobisher, who was eventually knighted and had an inlet of the Atlantic Ocean in northern Canada named after him—Frobisher Bay. The innermost point of the inlet of Frobisher Bay is the location of the Nunavut capital, formerly itself called Frobisher Bay, and now called Iqaluit.

It should be noted that the 1578 ceremony was not the first Thanksgiving as defined by First Nations tradition. Long before the time of Martin Frobisher, it was traditional in many First Nations cultures to offer an official giving of thanks during autumnal gatherings. In Haudenosaunee culture, Thanksgiving is a prayer recited to honor "The Three Sisters" (i.e., beans, corn, and squash) during the fall harvest.

In 1957, the Canadian Parliament declared Thanksgiving to be "a Day of General Thanksgiving to Almighty God for the bountiful harvest with which Canada has been blessed," and officially decided that the holiday take place on the second Monday in October.

While in the united states thanksgiving was first proclamaited by president washington on the 26th of november, and then was changed by President Lincoln declared Thanksgiving a Federal holiday as a "prayerful day of Thanksgiving" on the last Thursday in November. Since then every U.S. President has always made an official Thanksgiving Proclamation on behalf of the nation. President Franklin D. Roosevelt set the date for Thanksgiving to the fourth Thursday of November in 1939 (approved by Congress in 1941)