Great Union Day occurring on December 1, 1918, is the national holiday of Romania.

The year 1918 marks the unification of Transylvania, Bessarabia, and Bukovina with the Romanian Kingdom. This holiday was set after the Romanian Revolution and commemorates the assembly of the delegates of ethnic Romanians held in Alba Iulia, which declared the Union of Transylvania with Romania.

Prior to 1948, 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. In Communist Romania, the date of the national holiday was set to August 23 to mark the 1944 overthrow of the pro-fascist government of Marshal Ion Antonescu.

On December 1, 1918 (November 18 Old Style), the National Assembly of Romanians of Transylvania and Hungary, consisting of 1.228 elected representatives of the Romanians in Transylvania, Banat, Crişana and Maramureş, convened in Alba Iulia and decreed (by unanimous vote) the union with the Romanian Kingdom.
The Resolution voted by the National Assembly stipulated also the “fundamental principles for the foundation of the new Romanian State”. It was conditional, and demanded the preservation of a democratic local autonomy, the equality of all nationalities and religions.
The Assembly also formed from 200 of its members, plus 50 co-opted members a High National Romanian Council of Transylvania, the new permanent parliament of Transylvania.
The next day, on December 2, 1918, the High National Romanian Council of Transylvania formed a government under the name of Directory Council of Transylvania (Consiliul Dirigent al Transilvaniei), headed by Iuliu Maniu.
On December 11, 1918, King Ferdinand signed the Law regarding the Union of Transylvania, Banat, Crişana and Maramureş with the Old Kingdom of Romania.

Resolution no. 903 of the Council of Ministers on August 18, 1949 had marked August 23 as the national holiday. Law 10/1990, declared on August 1, 1990, moved the national holiday to December 1. The decision combated in some amount sympathy with the tradition of Romanian monarchy, associated with the day May 10, but also disappointed the anti-communist opposition, who wished for the national holiday to be moved to December 22.

The choice of December 1, though not explicitly declared in the law, referred to the unification of the provinces of Transylvania, Banat, Crişana and Maramureş with Romania in 1918.
Every year, on the Romanian National Day, an annual military parade on Constitution Square in Bucharest is held in honor of the occasion. A parade is also held in the city of Alba Iulia.

Into the year of 2018, the Romania celebrates the Centenary of the Great Union of all Romanians. The Romanian Orthodox Church proclamed this year as the Year of the Unity of the Faith and of the Nation.