Eastertide, Easter Sunday (Western)

Eastertide is the fifty day season this rule counts from Easter itself through Pentecost, and the Universal Norms governing the Roman Catholic liturgical calendar that this rule cites describe the whole stretch not as fifty separate days building toward a later feast but as a single continuous festival, traditionally called the Great Sunday, one extended celebration of the resurrection rather than an ordinary season punctuated by an ending. That framing has ancient roots, since writers as early as Tertullian in the late second and early third century already describe the fifty days between Easter and Pentecost, then generally called simply Pentecost after the Greek word for fiftieth, as a period during which fasting and kneeling in prayer were forbidden, treating the whole stretch as festal rather than penitential.

The number fifty itself echoes the Jewish counting of the Omer between Passover and Shavuot, another fifty day count between two related festivals, and the early church's fifty days similarly end at Pentecost, the feast of the Spirit's descent on the apostles. Within the season the Ascension, forty days after Easter, marks a distinct commemoration without ending the wider fifty day framing, and the whole season closes only when Pentecost itself arrives, at which point the white vestments of the Easter season give way to red, and the liturgical year moves into what the Roman calendar calls Ordinary Time.