Octave of Easter
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 (especially among Eastern Christians), Quasimodo Sunday and Quasimodogeniti. Since 1970 Low Sunday has been officially known as the Second Sunday of Easter (referring to the Easter season) in the Roman Catholic Church. On April 30, 2000, it was also designated as Divine Mercy Sunday by Pope John Paul II.
Octave refers to an eight-day feast following that feast where the feast is celebrated for a further 8 days. In the Roman Catholic Church Christmas also has an 8 day Octave similar to Easter.
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 1 Peter 2:2). Literally, quasi modo means "as if in [this] manner".
Western churches 
St. Thomas Sunday is so called because the Gospel reading always relates the story of "Doubting Thomas," in which Thomas the Apostle comes to believe in the Resurrection of Jesus only after being told by the resurrected Christ to place his finger in the nail marks and his hand in His side. In the Gospel accounts, this event takes place on the eighth day after the Resurrection, hence their significance for this Sunday (John 20:19-29).
The name "Low Sunday" is sometimes said to derive from its relative unimportance compared to the solemnities of Easter Day. Traditionally, the newly-baptised would receive baptismal gowns that would be worn until this day, and the official Latin name is Dominica in Albis [Depositis], "Sunday in [Setting Aside the] White Garments". Hence "White" and "Alb" Sunday—which is also the etymology of Whitsunday (Pentecost). Those who had been baptized at Easter laid aside the white robes which they had worn throughout the week. Long before the Reformation, confirmation was observed on this Sunday in Germany.
The Feast of the Divine Mercy 
Divine Mercy Sunday is the culmination of the novena to the Divine Mercy of Jesus, a devotion given to St. Faustina (Maria Faustina Kowalska) and is based upon an entry in her diary stating that anyone who participates in the Mass and receives the sacraments of Confession and the Eucharist on this day is assured by Jesus of full remission of their sins and punishments. The devotion was actively promoted by Pope John Paul II, who canonized St. Faustina and designated the Sunday after Easter as the Sunday of the Divine Mercy (Dominica II Paschae seu de divina misericordia) in the General Roman Calendar. John Paul II, who died in April 2005 on the vigil of Divine Mercy Sunday, was beatified on Divine Mercy Sunday, May 1, 2011, by his successor, Pope Benedict XVI.
Eastern churches 
According to the Synaxarion, "On this Sunday, the second Sunday of Pascha, we celebrate the Antipascha, that is to say the re-dedication of the Resurrection of Christ, and also commemorate the event of the Holy Apostle Thomas' touching the wounds of Christ."
Thomas Sunday and Renewal Sunday are other names by which this Sunday is known. The former name refers to the event commemorated that day, described in the Gospel passage read that day at the Divine Liturgy, (John 20:19-31), which recounts the story of Christ appearing to the Apostle Thomas in order to dispel the latter's doubt about the Resurrection. Among Eastern Christians Thomas is not so much remembered as "doubting Thomas," but is rather remembered for his confession of faith: "My Lord and my God," thus being the first to publicly proclaim the two natures of Christ: human and divine. The latter name reflects that on that day the celebration of the Resurrection is repeated and renewed.
The entire week from Pascha to Thomas Sunday, known as Bright Week, is considered to be one continuous day.
Literary note 
- Quasimodo, protagonist of the 1831 French novel Notre Dame de Paris (most often called in English The Hunchback of Notre Dame) by Victor Hugo, was found abandoned on the doorsteps of Notre Dame Cathedral on the Sunday after Easter, AD 1467 . In the words of the novel:
He [sc. archdeacon Claude Frollo, Quasimodo's adoptive father] baptized his adopted child and called him Quasimodo; whether it was that he chose thereby to commemorate the day when he had found him, or that he meant to mark by that name how incomplete and imperfectly molded the poor little creature was. Indeed, Quasimodo, one-eyed, hunchbacked, and bow-legged, could hardly be considered as anything more than an almost.
- The full line is "Quasi modo geniti infantes, rationabile, sine dolo lac concupiscite", from Catholic Encyclopedia listing for Low Sunday, roughly translated as "As newborn babes, desire the rational milk without guile..." Accessed November 18, 2009.
- Reed, Luther D. "The Lutheran Liturgy". Fortress Press, 1947. p. 509. ISBN 0-8006-0496-2
- A Divine Mercy Resource by Richard Torretto 2010 ISBN 1-4502-3236-1 pages 188-190
- "Pope paves way to beatification of John Paul II". bbc.news.co.uk. January 14, 2011. Retrieved 2011-01-14.
-  "Synaxarion: ANTIPASCHA - Thomas Sunday", Retrieved 2011-08-02
- Sokolof, Archpriest Dimitrii (1899), Manual of the Orthodox Church's Divine Services, Jordanville, New York: Holy Trinity Monastery (published 2001), p. 109, ISBN 0-88465-067-7
- Hugo, Victor (1996) . The Hunchback of Notre-Dame. Penguin Books. pp. 147–148. ISBN 978-0-14-062222-5.
- Thomas Sunday Orthodox icon and synaxarion
- "Low Sunday". Catholic Encyclopedia. New York: Robert Appleton Company. 1913.
|Sundays of the Roman Catholic liturgical calendar
Octave of Easter (Quasimodo Sunday)
2nd Sunday of Easter
3rd Sunday of Easter