This post contains Unicode Greek text. Internet Explorer will probably choke, so why not switch to Firefox and leave the Microsoft vale of sorrows behind? :-)
Ascension Day is the Christian holiday that commemorates the ascension of Jesus to heaven. The holiday is celebrated forty days after the day of the resurrection, so it is a movable holiday, changing from year to year in the calendar, although because Easter is always on Sunday, Ascension is always celebrated on a Thursday. In 2007, Ascension is celebrated on May 17, and Sunday May 22 is designated "Ascension Sunday."
Ascension is referred to in several passages in the New Testament, along with allusions in other passages (you can see a complete listing in the wikipedia article).
The verse in Mark (16:19) reads as follows (from the sheer joy of being able to cut and paste Unicode Greek into Blogger, I'll include both the Greek and English): ὁ μὲν οὗν κύριος ἰησοῦς μετὰ τὸ λαλῆσαι αὐτοῖς ἀνελήμφθη εἰς τὸν οὐρανὸν καὶ ἐκάθισεν ἐκ δεξιῶν τοῦ θεοῦ, "So Lord Jesus, after speaking to them, was received up into the sky (heaven) and he sat to the right of God."
Here is the verse in Luke (24:51): καὶ ἐγένετο ἐν τῶ εὐλογεῖν αὐτὸν αὐτοὺς διέστη ἀπ᾽ αὐτῶν καὶ ἀνεφέρετο εἰς τὸν οὐρανόν, "And it happened as he was blessing them, he stood apart from them and was carried up into the sky (heaven)."
The passage in Acts 1:9-11 provides more detail; here it is in the King James translation: "And when he had spoken these things, while they beheld, he was taken up; and a cloud received him out of their sight. And while they looked stedfastly toward heaven as he went up, behold, two men stood by them in white apparel; Which also said, Ye men of Galilee, why stand ye gazing up into heaven? this same Jesus, which is taken up from you into heaven, shall so come in like manner as ye have seen him go into heaven."
The ascension of Jesus is also a component of the Apostles' Creed and the Nicene Creed.
In the iconographic tradition for the Ascension scene, it is common to see simply the feet of Jesus as he is taken up. Here's an example from an illuminated manuscript from France dating to around the year 1200: