According to ITV:
Cannons were fired in public for the Queen, now the longest reigning monarch in British history.
It commemorates the start of her reign, which began on the death of her father, King George VI.
Princess Elizabeth, who was just 25, was thousands of miles away in Kenya…with the Duke of Edinburgh when she learned of his death. She returned home a queen.
Later in the year the Queen has a milestone birthday in April when she will turn 90, and a weekend of national events are planned to coincide with her official birthday celebrations in June.
