Special address by the British monarch
Special addresses by the monarch of the United Kingdom and other Commonwealth realms (and previously of the British Empire and its Dominions), outside the annual Royal Christmas Message and the Commonwealth Day Message, only take place at times of significant national or royal events. It was only in the 20th century, with the advent of radio, that the monarch was able to address all their subjects at once.
Source: Wikipedia — Special address by the British monarch (CC BY-SA 4.0)