The Easter Season lasts 50 Days. It starts with Easter Sunday, continues through the Octave of Easter, through the Ascension and ends on the Feast of Pentecost, when the Holy Spirit comes upon the apostles, Mary and the first followers of Christ.
"The 50 days from the Sunday of the Resurrection of Christ, to Pentacost Sunday are celebrated as one feast day, indeed as one 'Great Sunday'," according the the United States Conference of Catholic Bishops (USCCB). "These are the days above all others in which the Alleluia is sung."
The USCCB calls Easter "the most important of all liturgical times. It celebrates Jesus's victory over sin and death and salvation for mankind. It is God's greatest act of love to redeem mankind."
How long is the Easter Season in the Traditional Roman Rite?
In the traditional Roman Rite, the Easter Season was longer. It was made up of three seasons, the Season of Easter, Ascensiontide, and the Octave of Pentacost. The Season of Easter was from the Easter Vigil through the afternoon of the Vigil of the Ascension. Ascensiontide would begin the evening before Ascension with First Vespers of the Feast and end the afternoon of the Vigil of Pentacost - marking this time as a novena. The Octave of Pentacost is an extension of the Feast of Pentacost ending the following Saturday. The following Sunday being Trinity Sunday.