In February 1939, he received news from his sisters that his mother was dying. On <answer>10 February 1939</answer>, Pope Pius XI died. Roncalli was unable to see his mother for the end as the death of a pontiff meant that he would have to stay at his post until the election of a new pontiff. Unfortunately, she died on 20 February 1939, during the nine days of mourning for the late Pius XI. He was sent a letter by Cardinal Eugenio Pacelli, and Roncalli later recalled that it was probably the last letter Pacelli sent until his election as Pope Pius XII on 2 March 1939. Roncalli expressed happiness that Pacelli was elected, and, on radio, listened to the coronation of the new pontiff. 
When did Pope Pius XI die?
10 February 1939