Eastertide, or the Easter Season, begins on Easter Sunday and continues until Pentecost in the Christian liturgical calendar, thus spanning a total of seven weeks. Some denominations — most notably the Roman Catholic Church and the Anglican churches — formerly included the next two weeks as well.

Until 1970, the Roman Catholic calendar labelled the Sundays following Easter Sunday as "Sundays After Easter," the first such Sunday often being called Low Sunday, the next Sunday the Second Sunday After Easter, the Sunday after that the Third Sunday After Easter, and so on. The fifth Sunday after Easter was sometimes called Rogation Sunday, or "the Sunday before the Rogation days." On the Thursday after the aforementioned Sunday, forty days after Easter Sunday, is the feast of the Ascension, and the Sunday falling three days after this was known as the "Sunday After Ascension" and not the "Sixth Sunday After Easter." Pentecost is the next Sunday, followed by Trinity Sunday, and four days after the latter, the feast of Corpus Christi. The calendar week (Sunday through Saturday) beginning on Trinity Sunday was deemed the last week of the Easter season, which thus encompassed nine weeks.

The new calendar which took effect in 1970 following its earlier approval by the Second Vatican Council changed the "Sundays after Easter" to "Sundays of Easter," with Low Sunday becoming the "Second Sunday of Easter," the next Sunday the "Third Sunday of Easter," etc., with the Sunday after the Ascension being renamed the "Seventh Sunday of Easter." While Pentecost and Trinity Sunday were themselves retained, the entire weeks starting with these Sundays were no longer considered part of the Easter season, instead being reckoned as the first two weeks within the second installment of Ordinary Time. Concomitantly, red vestments, which had been authorized for the entire week of Pentecost prior to the calendar reform, were to henceforth be used on the day of Pentecost only; similarly, white vestments continued to be used on Trinity Sunday itself, but the liturgical color became green for both weeks other than the Sundays. In addition, in the United States only, the feast of Corpus Christi was moved three days forward, to the Sunday after Trinity Sunday, when it had heretofore been celebrated on the Thursday after Trinity Sunday.

When the Anglican churches implemented their own calendar reform effective in 1976, they adopted the same shortened definition of the Easter season as the Roman Catholic Church had promulgated six years earlier. Some Anglican provinces continue to label the Sundays between Easter and the Ascension "Sundays After Easter" rather than "Sundays of Easter"; however, others, such as the Episcopal Church in the United States of America, use the term "Sundays of Easter".