Queen's birthday message: This year it's 'difficult to escape a very somber national mood'

— -- Saturday marks Queen Elizabeth II's official birthday -- her actual 91st birthday was April 21 -- but the reigning monarch said it's difficult to be in a celebratory mood in light of recent terror attacks in London and Manchester.

"Today is traditionally a day of celebration," reads a message from the Queen released by Buckingham Palace Saturday morning. "This year, however, it is difficult to escape a very sombre national mood. In recent months, the country has witnessed a succession of terrible tragedies. As a nation, we continue to reflect and pray for all those who have been directly affected by these events."

She continues, "During recent visits in Manchester and London, I have been profoundly struck by the immediate inclination of people throughout the country to offer comfort and support to those in desperate need."

The message concludes, "Put to the test, the United Kingdom has been resolute in the face of adversity. United in our sadness, we are equally determined, without fear or favour, to support all those rebuilding lives so horribly affected by injury and loss."

The Queen visited the Royal Manchester Children's Hospital following the attack at Manchester Arena during an Ariana Grande concert, which left 22 people dead. And she and The Prince of Wales earlier this month attended church services, where they prayed for the victims of the London Bridge attack, which resulted in seven deaths.

As for why the Queen celebrates her birthday twice, the Royal Mint explains, "The official birthday is held in the summer as it means there’s a better chance of good weather for the ceremony." It's a tradition that began with King George II in 1748.

The Queen's official birthday is celebrated with the Birthday Parade, which is also known as Trooping the Colour. During the ceremony, troops from the Household Division greet The Queen with a Royal salute. She then inspects the troops, riding past them in a carriage.