The question of when Transylvania became a part of the Kingdom of Romania, is far more interesting: On December 1, 1918 a vote took place in the city of Alba Iulia (near a the ruins of its citadel). The people of Transylvania voted on that day to join with the Kingdom of Romania, so this is one possible date.
King Ferdinand of Romania signed a new constitution in 1923 that dealt with Transylvania and the other territories gained after the Paris Peace conferences. There are then several dates on which one could say Transylvania became a part of Romania. The most accepted is in 1918 and the Paris Peace conference of 1919.