The fifty days of Eastertide are inherited arithmetic. In the Torah, Israel is commanded to count seven full weeks from the offering of the first sheaf of the barley harvest at Passover, and to keep the fiftieth day as a feast: the counting of the Omer, ending at Shavuot, the feast of weeks. Greek speaking Jews called that fiftieth day Pentecost, and the Christian season took over the frame entire, counting its fifty days from Pascha, the Christian Passover, to a fiftieth day feast that kept the borrowed name. The church filled the frame with its own content.
Shavuot celebrated the giving of the covenant; the Christian Pentecost celebrates the descent of the Spirit on the apostles, an event the Acts of the Apostles places precisely on that Jewish feast, when Jerusalem was full of pilgrims. The parallel was not lost on the tradition, which read the two fiftieth days against each other, law and Spirit. One habit of the season, the count itself, has kept devotional force in both traditions: as Jewish practice counts the Omer aloud each evening, Christian communities have kept the sense of numbered days moving toward a completion, with the Ascension marking day forty and the final novena of waiting closing the season. The season is thus one of the clearest places where the Christian calendar shows its Jewish bones.