Context: In 1918, the castle became the seat of the president of the new Czechoslovak Republic, T.G. Masaryk. The New Royal Palace and the gardens were renovated by Slovenian architect Jože Plečnik. In this period the St Vitus Cathedral was finished .  Renovations continued in 1936 under Plečnik's successor Pavel Janák. On March 15, 1939, shortly after the Nazi Germany forced Czech President Emil Hacha  to hand his nation over to the Germans, Adolf Hitler spent a night in the Prague Castle, "proudly surveying his new possession." During the Nazi occupation of Czechoslovakia in World War II, Prague Castle became the headquarters of Reinhard Heydrich, the Reich Protector of Bohemia and Moravia. According to a popular rumor, he is said to have placed the Bohemian crown on his head; old legends say a usurper who places the crown on his head is doomed to die within a year. Less than a year after assuming power, on May 27, 1942, Heydrich was attacked during Operation Anthropoid, by British-trained Slovak and Czech soldiers while on his way to the Castle, and died of his wounds - which became infected - a week later. Klaus, his firstborn son, died the next year in a traffic accident, also in line with the legend. After the liberation of Czechoslovakia and the coup in 1948, the Castle housed the offices of the communist Czechoslovak government. After Czechoslovakia split into the Czech Republic and Slovakia, the castle became the seat of the Head of State of the new Czech Republic.  Similar to what Masaryk did with Plečnik, president Václav Havel commissioned Bořek Šípek to be the architect of post-communism Prague Castle's necessary improvements, in particular of the facelift of the castle's gallery of paintings.

Question: When did the castle become the seat of the Head of State of the new Czech Republic?

Answer:
Czechoslovakia split into the Czech Republic and Slovakia