Nunavut Land Claims Agreement Act and the Nunavut Act, which established the intention for Nunavut to be a distinct territory at some point. On April 1, 1999, this became a reality and Nunavut was officially made a territory. Nunavut Day celebrates the original passing of the Nunavut Act, which began the territories journey to establishment.
Nunavut is over two million square kilometers and makes up 20% of Canada's land mass. As of 2011, there are approximately 33,330 people living in Nunavut the majority of which are Inuit peoples. The creation of Nunavut marked a significant milestone for Canada;s Inuit people -- as the creation of Nunavut marked the beginning of self-governance of the Inuit.
Also, as far as I know Nunavut is the only province or territory in Canada that has an official holiday to celebrate its founding, heritage, and culture. The day emphasizes the significance of the creation of this territory and also serves to remind residents and other Canadian's of the long debate and struggle undertaken in the establishment of Nunavut.