@Mr_M: Think of him as just any other researcher. He is part of a team that operates the Vatican Observatory, which, if you look on their website is one of the oldest astronomical research institutes in the world. It says that is was initiated to help with the reformatting of the calendar in the 1500s. The Vatican just oversees it. So I’d imagine that when the people that work there want to go on vacation, it works just like any other research institute. They study astronomy.
Remember that many of the holidays recognized by the church follow lunar and celestial guidelines (like Easter). So it was important to have someone who could tell them when the first full moon of the third month occurred (or something like that).