It is a tradition that was started by George II in 1748 and it owes its origins to the ageless problem of the British weather. George was born in November and felt the weather would be too cold for his annual birthday parade. Instead, he combined his birthday celebration with an annual spring military parade known as Trooping the Colour. It is a tradition that has continued to this day.
Why is her official birthday always on Saturday?
During the early part of her reign, the Queen's official birthday was on a Thursday, but it later changed to a Saturday in June so that more members of the public could enjoy it."