Princess Anne, the only daughter of the late Queen Elizabeth II and Prince Philip, is preparing to mark her 75th birthday on Friday, 15 August.

Despite the milestone, the Princess Royal has a very particular rule when it comes to birthday celebrations — and it’s one she’s followed for years.

Roya Nikkhah, royal correspondent for the Sunday Times, revealed that Anne takes a “no-frills” approach to marking her special day. The rule is simple: she only celebrates birthdays ending in a zero.

“Anne’s policy is: ‘I will do things for my birthdays that have a zero, but I won’t do things for my birthdays that have a five’,” Nikkhah explained.