The order members celebrate their birthdays in a completely different way than humans do. For a member of the order a year in their native world is the equivalent to 10 years in the human world and this means that when a human year passes they pretend to have a birthday like all humans and celebrate in a human style, but every decade they travel outside of planet Earth and visit their own planet in order to celebrate with their kind.
The birthday of a member of the order is a huge event on their home planet, but they do not celebrate with drinks, music and food. Their celebrations are a series of rituals that are far beyond the comprehension of a human being. The order are never going to celebrate their birthdays on Earth because the planet doesn't have the conditions for this to take place.