A calendar looks like an administrative object. It is not. It is a claim about which moments are thick enough to return to.
Every tradition that lasts long enough to be called a religion keeps time on purpose. Fast days. Feast days. A week that is not merely seven mornings. A year that is not merely an orbit. The point is not to decorate the clock. The point is to refuse the idea that all hours are equal.
Modern life tends the other way. The hour becomes a unit of production. Days blur. The weekend is a recovery interval, not a different kind of time. In that weather it is easy to think of liturgy as costume — something extra people put on when they want a feeling. The older reading is inverted. The form is what makes a people able to feel the same thing together, later, when the original mood is gone.
This is why a holy day can survive the death of the person who first kept it. The calendar is a memory that does not require a single rememberer. It is also a philosophy of the body: when to eat, when to refrain, when to gather, when to be quiet in public. Those are metaphysical instructions wearing the clothes of a schedule.
None of this requires you to accept every inherited date. Calendars have been used to conscript as well as to keep. The useful question is smaller. What, in a life, is allowed to recur? What is given a name so it cannot be swallowed by the next task?
Religion, on this site, is the thread that takes that question seriously — not as private preference, but as a form a people can inhabit together. Spirituality may arrive as silence. Religion arrives as a date on the wall, and the decision to treat it as more than ink.