When the pilgrims arrived in what is currently Massachusetts, they endured a harsh first winter, however they were fortunate enough to be aided by Tisquantum, also known as Squanto. Through his aid, the pilgrims survived and the colony flourished, which led to further colonization of the area, which led to it becoming one of the most important locations of the American Revolution. Tisquantum played in import role in early American history, and he should be honored as such.
If you have to make room for it, you can always get rid of Columbus Day. A lot of people aren't too keen on Columbus anymore, and he didn't really even make it all the way to the America, he discovered the West Indies (The Caribbean). So anyway, you could put Tisquantum day there and everything would be all good.