Eastertide

Eastertide is the English term that means Easter Season. The season of Easter consists of 50 days, from Easter Sunday (March 23rd, 2008) to Pentecost Sunday (May 11th, 2008).
We then return to Ordinary Time.

The Fifty days from Easter Sunday to Pentecost are celebrated in joyful exultation as one feast day, or better as one "great Sunday."

These above all others are the days for the singing of the Alleluia.

The Sundays of this season rank as the paschal Sundays and, after Easter Sunday itself, are called the Second, Third, Fourth, Fifth, Sixth, and Seventh Sundays of Easter. The period of fifty sacred days ends on Pentecost Sunday. The first eight days of the Easter Season make up the octave of Easter and are celebrated as solemnities of the Lord.

On the fortieth day after Easter the Ascension is celebrated, except in places where, not being a holy day of obligation, it has been transferred to the Seventh Sunday of Easter. This solemnity directs our attention to Christ, who ascended into heaven before the eyes of his disciples, who is now seated at the right hand of the Father, invested with royal power, who is there to prepare a place for its in the kingdoms of heaven; and who is destined to come again at the end of time.

The weekdays after the Ascension until the Saturday before Pentecost inclusive are a preparation for the coming of the Holy Spirit, the Paraclete.

This sacred season of fifty days comes to an end on Pentecost Sunday, which commemorates the giving of the Holy Spirit to the apostles, the beginnings of the Church and its mission to every tongue and people and nation. (Excerpted from the General Norms for the Liturgical Year and Ceremonial of Bishops)

sources: Catholic Culture's Easter Workshop Catholic Culture website
General Norms for the Liturgical Year and Ceremonial of Bishops