Today's feast commemorates the ascension of Jesus into heaven, after having been with his disciples "forty days" (Acts 1: 3) after His resurrection. The feast was instituted in the fourth century by the church in Jerusalem, from which it spread to the church as a whole.
Prior to the 4th century, the Ascension was celebrated - along with the Resurrection and the gifts of the Holy Spirit given on Whitsunday (Pentecost) - as part of the fifty day Eastertide commemoration of mankind's redemption through the saving acts of Jesus.
However, in our day, those individual events now have their specific and unique celebrations - Eastertide (40 days from Easter onwards), Ascensiontide (10 days from Ascension Day to the day before Pentecost/Whitsunday), and Whitsuntide (the week of Pentecost, or Whitsunday).
The above picture - an oil on canvas - is by Rembrandt, and was done in 1636.
