Forty days after the Resurrection of Our Divine Lord, He Ascended into Heaven, we read in the Acts of the Apostles 1:1-11:
The former treatise I made, O Theophilus, of all things which Jesus began to do and to teach, Until the day on which, giving commandments by the Holy Ghost to the apostles whom he had chosen, he was taken up. To whom also he shewed himself alive after his passion, by many proofs, for forty days appearing to them, and speaking of the kingdom of God. And eating together with them, he commanded them, that they should not depart from Jerusalem, but should wait for the promise of the Father, which you have heard (saith he) by my mouth. For John indeed baptized with water, but you shall be baptized with the Holy Ghost, not many days hence.
They therefore who were come together, asked him, saying: Lord, wilt thou at this time restore again the kingdom to Israel? But he said to them: It is not for you to know the times or moments, which the Father hath put in his own power: But you shall receive the power of the Holy Ghost coming upon you, and you shall be witnesses unto me in Jerusalem, and in all Judea, and Samaria, and even to the uttermost part of the earth. And when he had said these things, while they looked on, he was raised up: and a cloud received him out of their sight. And while they were beholding him going up to heaven, behold two men stood by them in white garments. Who also said: Ye men of Galilee, why stand you looking up to heaven? This Jesus who is taken up from you into heaven, shall so come, as you have seen him going into heaven. Then they returned to Jerusalem...
For more details and customs associated with this great and high feast of Ascension Thursday, you can't do much better than this site:
Of course, the site of this event is the great Mount of Olives just across the valley from the old city of Jerusalem. Most specifically, the Chapel of the Ascension: http://www.sacred-destinations.com/israel/jerusalem-chapel-of-ascension
In most of the United States, the obligation and observation of this great feast is transferred to the following Sundays -- except those in the ecclesiastical provinces of Boston (MA, ME, NH, VT), Hartford (CT, RI), New York City (NY), Newark (NJ), Philadelphia (PA), and Omaha (NE).
Still, happy Feast! Live well!
Ascension of Christ by Garofalo (1520AD)