Well, the national day of Romania is about the unification of all three romanian inhabitated regions, Transylvania, Vallachia and Moldavia in Greather Romania.

Usually the day starts with a millitary parade, and people gather arround in Alba Iulia (the unification town) or Bucharest (the capital) to watch the parade and gather togeder to celebrate the national unification day. After that you could think of some sort of thematical unification park, like the Christmas parks around the world, where people strawl arround, enjoy the specific cuisine and celebrate the romanian culture, with songs, various reenactions, and some concerts.

LE: also the day is a national celebration day, where most of the people have a day off to rest and to be around their families.

Well, you are right, no one goes to picnics in december, but people in Romania visit their family and they celebrate the reunification day togeder.