Ascension Day is celebrated in the U.S. on the 40th day of Easter. The New Testament states during the 40 days after his resurrection Jesus Christ met with his disciples instructing them how to carry on his teachings.
It is believed that on the 40th day he took them to the Mount of Olives, where they watched as he ascended to heaven. 
Ascension Day marks the end of the Easter season and occurs ten days before Pentecost. Penecost is a prominent feast in the calendar of Ancient Israel celebrating the giving of the Law on Sinai, and also later in the Christian liturgical year commemorating the descent of the Holy Spirit upon the remaining eleven Apostles of Christ (Judas had hanged himself), included in the 12 Disciples in the Upper Room, after the Resurrection of Jesus. 
Depending upon the phases of the moon in a particular year, Ascension Day is celebrated on a Thursday. However, some churches, particularly in the United States, celebrate it on the following Sunday. 
 Ascension Day in United States