Great Union Day (Romanian: Ziua Marii Uniri, also called Unification Day) 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 Romanian Revolution (1989) and it marks the unification of Transylvania, but also of the provinces of Bessarabia and Bukovina with the Romanian Kingdom, in 1918.
Prior to 1948 (Communist era), the national holiday of Romania was set to be on May 10, which had a double meaning: it was the day on which Carol I set foot on the Romanian soil (in 1866), and it was the day on which the prince ratified the Declaration of Independence (from the Ottoman Empire) in 1877. (Wikipedia)