Although there have been fan gatherings on other dates, the only one called "official" is on October 24th.
If there is not one near you, organize one. A kind of tradition is to submit a group photo after the APH Day to the main organizers, which normally make a slideshow to show that Hetalia fans are friends all around the word, no matter in which country they live, or how old they are.
Many fans see the Hetalia Day as a symbol for world peace too.
Fans typically cosplay at these events, but whether or not it is required depends on the wants of the group you are meeting with.
Locations for the 2010 event
Please visit the link below for the locations: