The fifty days from the Sunday of the Resurrection to Pentecost Sunday are celebrated in joy and exultation as one feast day, indeed, as one "great Sunday" (St. Athanasius, Epistula. festalis: PG26, 1366).
These are the days above all others in which the Alleluia is sung.
The Sundays of this time of year are considered to be Sundays of Easter and are called, after Easter Sunday itself, the Second, Third, Fourth, Fifth, Sixth, and Seventh Sundays of Easter. This sacred period of fifty days concludes with Pentecost Sunday.
The first eight days of Easter Time constitute the Octave of Easter and are celebrated as Solemnities of the Lord.
--(Universal Norms on the Liturgical Year and the Calendar, nos.22-24)