In February, King Charles took a break from his royal duties after being diagnosed with cancer. By the end of April, he was back at work.
The problem is trying to stop him, Queen Camilla has said several times about her husband.
The birthday is celebrated with traditional salutes, but the king does not take the day off to celebrate. On Thursday, he will attend the opening of a center for distributing surplus food, part of an initiative launched on his 75th birthday.
The past year has been worrying for the British royal family. Son Prince Harry has moved with his family to the USA, and both Charles' and William's private fortunes have recently been the subject of a critical documentary.
Most worrying, however, have been King Charles' and 42-year-old daughter-in-law Catherine's cancer diagnoses.
To be honest, it has been terrible. It has been probably the toughest year of my life, said Prince William recently during a trip to South Africa.
