The Extraordinary Form treats Ascensiontide (the period from the Ascension until Pentecost) as a separate liturgical season, while the Ordinary Form considers it part of Paschaltide. With regard to fulfillment of the Old Covenant, Christ rose from the dead the day the first sheaf is offered before the Lord in the Temple, and the Holy Spirit descended on the Apostles seven weeks later, at the anniversary of the Giving of the Law at Sinai, which is called Pentecost or Shavuot. At this time, a first fruit offering of leavened bread is offered before the Lord in the Temple.
Thus, the Ordinary Form more accurately reflects the fulfillment of the Old Covenant, but the Extraordinary Form has perfectly valid reasons for treating Ascensiontide as a separate season. The Ascension of the Lord is classed as a joyful feast, but it is in some ways bittersweet: We rejoice at Christ being enthroned as King of the Universe, and we await the Coming of the Spirit, but we also long to see Him face-to-face. Immediately after the Ascension, the angels told the Apostles to look forward to the Second Coming, which we continue to do to this day.
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