The practice of observing an eight day period, or octave, of celebration for major liturgical feasts has its roots in the Feast of Tabernacles in the Old Testament (Leviticus 23:36) and with Constantine I circa 300 A.D with the dedication festivities of the basilicas at Jerusalem and Tyre.
There have been many octaves celebrated in the Catholic Church, but since the 1955 decree by Pope Pius XII the Roman calendar only observes 3: Christmas, Easter, and Pentecost.
[Wikipedia] The term Octave of Easter may refer either to the eight day period (Octave) from Easter Sunday until the Sunday following Easter, inclusive; or it may refer only to that Sunday after Easter, the Octave Day of Easter (sometimes known as Low Sunday). That Sunday is also known historically as St. Thomas Sunday , [Divine Mercy Sunday], and Quasimodo Sunday.
The name Quasimodo came from the Latin text of the traditional Introit for this day, which begins "Quasi modo geniti infantes..." ("As newborn babies...", from the First Epistle of Peter (I_Peter 2:2).  Literally, quasi modo means "as if in [this] manner".(And yes, the Hunchback of Notre Dame takes his name from this day.)
The Global Chant Database lists several other arrangements for the Introit for Quasimodo Sunday.
The full Introit line is translated, "[L]ike newborn infants, long for pure spiritual milk so that through it you may grow into salvation."