Although in fact that's not quite true. If the sovereign is born in the summer months (and I don't know just what constitutes 'the summer months') he or she has just one birthday each year.
This business of having two birthdays goes back more than 250 years. Back in 1748, the King was George II. He had been born in November and he decided that the weather at that time of the year would be too cold, and possibly too wet, for his birthday parade. He had the idea of combining his birthday celebration with a military parade held in the spring each year.
And so it has been ever since.
That spring military parade is still held each year - it was held this morning - and it is known as Trooping the Colour. Each year, one of the regiments of the Household Brigade - the five Guards regiments, being the Coldstream, the Grenadier, the Irish, the Scots and the Welsh Guards - parades its colour - the flag - in front of the entire regiment. This was originally done so that the soldiers would recognise their colours when in battle. These days it is purely a ceremonial affair which combines the trooping (parading) of the colours with a royal inspection.
Here is a clip of the BBC broadcast of this morning's parade: