The first hetalia day was in october 2009. Although there have been fangatherings on other dates and occasions with the same name the only "official" gatherings are held on the 24th of october.
If there is none near you, you can organize one.
The reason behind october 24th is the United Nations Day.
Fans usually cosplay at these evens but it is not a must.
Locations for the 2010 event
Please visit the link below for the locations: