National Tartan Day is the day on which people of Scottish heritage celebrate with Scottish-themed feasting and entertainment. In Scotland itself, it is a week-long event.
Tartan Day is celebrated on April 6th because that was the date on which the Scottish declaration of independence, the Declaration of Arbroath, was signed in 1320. (That was when it was signed. It took the English another eight years to get the message and actually recognize Scotland’s independence.)
Scottish clans all have their own tartans, but Canada’s official tartan is the Maple Leaf Tartan. Though made official by ministerial declaration on March 9, 2011, it has long been regarded as such. It was actually created by David Weiser in 1964 in readiness for Canada’s 1967 confederation centennial. Canadians of all ethnic backgrounds are allowed to wear it to declare their nationality, especially on Tartan Day, and Canada Day (July 1st). The basis for the design was the colour changes maple leaves go through during the year: green in summer, gold in autumn, red with the first frost, and brown once fallen.