The Easter Vigil is the "Mother of All Vigils." Easter Sunday, then, is the greatest of all Sundays, and Easter Time is the most important of all liturgical times.  Easter is the celebration of the Lord's Resurrection from the dead, culminating in his Ascension to the Father and sending of the Holy Spirit upon the Church on Pentecost.

There are 50 days of Easter from the first Sunday of the Resurrection to Pentecost Sunday. It is characterized, above all, by the joy of glorified life and the victory over death, expressed most fully in the great resounding cry of the Christian: Alleluia! All faith flows from faith in the resurrection: 

"If Christ has not been raised, then empty is our preaching; empty, too, is your faith."
(1 Corinthians 15: 14)

"What you sow is not brought to life unless it dies. And what you sow is not the body that is to be, but a bare kernel of wheat, perhaps, or of some other kind;…

So also is the resurrection of the dead.  It is sown corruptible; it is raised incorruptible. It is sown dishonorable; it is raised glorious. It is sown weak; it is raised powerful. It is sown a natural body; it is raised a spiritual body. If there is a natural body, there is also a spiritual one.

So, too, it is written, "The first man, Adam, became a living being," the last Adam a life-giving spirit. But the spiritual was not first; rather the natural and then the spiritual. The first man was from the earth, earthly; the second man, from heaven.

As was the earthly one, so also are the earthly, and as is the heavenly one, so also are the heavenly. Just as we have borne the image of the earthly one, we shall also bear the image of the heavenly one." 

(1 Corintians 15: 36-37, 42-49)