The Feast of the Ascension commemorates the day Jesus ascended into heaven, 40 days after His Resurrection. On that day, Christ took His disciples to the Mount of Olives and blessed them, telling them not to leave Jerusalem until the Holy Spirit came to them.
The Bible tells of Christ's ascension into heaven in Mark 16, Luke 24 and Acts 1. After Jesus was out of the disciples' sight, an angel came to them and told them that just as Christ ascended, He would return.
The Feast of the Ascension does not come on the same day every year because Easter's date changes, and the Church always celebrates the Feast of the Ascension 40 days after Easter. The Feast of the Ascension is always on a Thursday.