Ascension Icon by Andrei Rublev (late 14th - early 15th century).
The Ascension of Our Lord Jesus Christ is traditionally celebrated, following the detail of Acts 1:3, on the Thursday 40 days after Pascha (Easter), which was last Thursday, though most Latin Rite dioceses (at least in the U.S., if not in other countries as well) transfer the celebration of this feast to the next Sunday, which is today. In Greek, this feast is called 'Η Αναληψις: analepsis means a "taking up," although the term is used in narrative analysis (of films, novels, and other works) to indicate a flashback.
The events of this feast are outlined in Mark, Luke, and Acts:
"So then the Lord Jesus, after He had spoken to them, was taken up into Heaven, and sat down at the right hand of God" (Mk 16:19).
"Then He led them out as far as Bethany, and lifting up His hands He blessed them. While He blessed them, He parted from them, and was carried up into Heaven" (Lk 24:50-51).
"And when He had said this, as they were looking on, He was lifted up, and a cloud took Him out of their sight" (Acts 1:9).
The Ascension of Our Lord by the Novgorod School, in the Malo-Kirillov Monastery (1543).
The festal icon incorporates some of the details of these accounts, but it is more meant to be a representation of the Church than the event per se. The main figure is, of course, Our Lord Jesus Christ in a mandorla (though it is usually more round than almond-shaped) flanked by angels, giving a blessing with His right hand (as Luke relates) and holding a scroll in His left hand. Though the icon is of the Ascension, there is no directional movement explicitly depicted, so it could also represent His Second Coming (since, as angels told the apostles, "This Jesus, Who was taken up from you into Heaven, will come in the same way as you saw Him go into Heaven" (Acts 1:11)).
Below the Lord is the Theotokos, Mary the Mother of God, flanked by angels and surrounded by disciples. The icon is not strictly historical, thus Mary is present, as is St. Paul--usually found just to the right of the Blessed Mother--who had not been converted yet. While the disciples are in confusion at this event, the Theotokos is calm and in a posture of prayer. She is in the center of the icon, below Jesus. Mary together with the disciples represent the entire Church as she celebrates the Lord's Ascension and awaits His Second Coming. The angels in white garments both represent the angels who appeared to the disciples after the Ascension (vid. Acts 1:10-11) and the angels who constantly guard the Church at the Lord's command.
Ascension Icon by a Coptic Christian artist (14th century).
The Ascension by Christo Dimitrov of the Samokov School,
in the Church of Ss. Cyril and Methodius in Gabrovnitsa, Pazardjik, Bulgaria (19th century).
"May Almighty God bless you,
for on this very day His Only-Begotten Son
pierced the heights of Heaven
and unlocked for you the way
to ascend to where He is.
--Roman Missal, Solemn Blessing 7: The Ascension of the Lord
The Ascension of Christ by Pietro Perugino (late 1490s).
[Though not a traditional icon, it follows the same general structure.]
The Ascension by Theophanes the Cretan, in Stavronikita Monastery on Mount Athos (1546).
"After fulfilling for us Your Plan of Redemption
and joining the things of earth with those of Heaven,
You gloriously ascended, O Christ our God,
without abandoning us, but remained with us forever
and proclaimed to those who love You:
'Behold, I am with you;
no one has power against you.'"
--Kontakion of the Ascension of our Lord, God and Savior Jesus Christ (6th Tone)
Ascension icon on Folio 13v of a Syriac Gospel book by the Scribe Rabbula
from the Monastery of St. John of Zagba (586).
[Information found here.]
Ascension icon from Sinai (9th century).
O All-Blameless Theotokos, pray for us to your Son, Our Lord Jesus Christ, the Ascended One!
Nota Bene: Biblical quotations are all from the RSV-CE published by Ignatius Press. Quotation from the Roman Missal was found at CNP Liturgical Planning. The Kontakion was found in the Publican's Prayer Book published by the Melkite Catholic Eparchy of Newton. Other sources of information for this post were OrthodoxWiki, the Greek Orthodox Archdiocese of America, and Answers.com.