The Ascension of Our Lord, which occurred 40 days after Jesus Christ rose from the dead on Easter, is the final act of our redemption that Christ began on Good Friday. On this day, the risen Christ, in the sight of His apostles, ascended bodily into Heaven (Luke 24:51; Mark 16:19; Acts 1:9-11).
Like the dates of most other moveable feasts, the date of the Ascension depends on the date of Easter. Ascension Thursday always falls 40 days after Easter (counting both Easter and Ascension Thursday), but since the date of Easter changes every year, the date of Ascension does as well. (See How Is the Date of Easter Calculated? for more details.)
Determining the date of Ascension is also complicated by the fact that, in many dioceses of the United States (or, more accurately, many ecclesiastical provinces), the celebration of Ascension has been transferred from Ascension Thursday (40 days after Easter) to the following Sunday (43 days after Easter).
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Image: “The Ascension of Christ” by Giovanni Domenico Tiepolo (circa 1745-1750)