Hetalia Day is a recognized by Hetalia fans as a worldwide fan gathering held on the 24th of every October, in whichever town or city a group of fans agree to meet in. The date was set because of, and coincides with, United Nations Day. The first Hetalia day was in October of 2009.
Although there have been fan gatherings on other dates, of course, but the only one called "official" is held on October 24th.
If there is not one near you, organize one.
Fans typically cosplay at these events, but whether or not it is required depends on the wants of the group you are meeting with.