Answer:
1) Former President Benjamin Harrison. After the murder of 11 italian immigrants, he declared it as an one-time national celebration, in order to ease diplomatic tensions with Italy.
2) Indigenous people Day. Since 2015, several Council members from the District of Columbia have tried to change the name, which had happened this year. Now, the District of Columbia celebrates "Indigenous People Day".
3, 4) 1451, in Genova.
5) 29 days. The crew spotted a flock of birds, which were a sign of land.
6) On his first voyage, between 86 to 89.
7) 1792.
8) New York City with over 35,000 marchers and one million viewers.
9) Italian.
10) Puerto Rico Friendship Day.