The earliest documents mention that Christians meet statu die—on a fixed day—and nothing in the long history of Christianity could alter the importance of this fixed day.
If Christianity were a purely “spiritual” and eschatological faith there would have been no need for a “fixed day,” because mysticism has no interest in time. To save one’s soul one needs, indeed, no “calendar.” And if Christianity were but a new “religion,” it would have established its calendar, with the usual opposition between the “holy days” and the “profane days”—those to be “kept” and “observed” and those religiously insignificant. Both understandings did in fact appear later. But this was not at all the original meaning of the “fixed day.” It was not meant to be a “holy day” opposed to profane ones, a commemoration in time of a past event. Its true meaning was in the transformation of time, not of calendar. For, on the one hand, Sunday remained one of the days (for more than three centuries it was not even a day of rest), the first of the week, fully belonging to this world. Yet on the other hand, on that day, through the eucharistic ascension, the Day of the Lord was revealed and manifested in all its glory and transforming power as the end of this world, as the beginning of the world to come. And thus through that one day all days, all time were transformed into times of remembrance and expectation, remembrance of the ascension, (“we have seen the true light”) and expectation of its coming. All days, all hours were now referred to this end of all “natural” life, to the beginning of the new life. The week was no longer a sequence of “profane” days, with rest on the “sacred” day at their end. . . . Sunday . . . was not a “sacred” day to be “observed” apart from all other days and opposed to them. It did not interrupt time with a “timeless” mystical ecstasy. It was not a “break” in an otherwise meaningless sequence of days and nights. By remaining one of the ordinary days, and yet by revealing itself through the Eucharist as the eighth and first day, it gave all days their true meaning. It made the time of this world a time of the end, and it made it also the time of the beginning.
—Alexander Schmemann, For the Life of the World, 51-52