Great Union Day (Romanian: Ziua Marii Uniri, also called Unification Day) occurring on December 1, is the national holiday of Romania.

"Great Union Day (Romanian: Ziua Marii Uniri, also called Unification Day^[1]) occurring on December 1, is the national holiday of Romania. It commemorates the assembly of the delegates of ethnic Romanians held in Alba Iulia, which declared the Union of Transylvania with Romania.

This holiday was set after the 1989 Romanian Revolution and it marks the unification of Transylvania, but also of the provinces of Bessarabia and Bukovina with the Romanian Kingdom, in 1918."

http://en.wikipedia.org/wiki/Great_Union_Day

A previous Unification was accomplished by Michael The Brave, in 1600:

"Michael the Brave (Romanian: Mihai Viteazu(l), Mihai Bravu, 1558 - August 9, 1601) was the Prince of Wallachia (1593–1601), of Transylvania (1599–1600), and of Moldavia (1600), the three Romanian principalities^{note 1} being joined in the form of personal union for a brief time under his rule."

http://en.wikipedia.org/wiki/Michael_the_Brave