Easter Sunday is the third day of the Sacred Paschal Triduum and also the first Sunday of the Easter season. The very first Christian centuries had no liturgical celebration on this day, except the Easter vigil, which only would reach conclusion early in the morning on Sunday with the celebration of the Eucharist. However, already the pilgrim Egeria (or Etheria) (a 4-th century pilgrim from Western Europe all the way to the Holy Land) reports about a second Eucharist on Easter Sunday in Jerusalem. The church father Augustine also knows a second Eucharist on Sunday morning. In addition, there was also a memorial of the apparition of Jesus on Sunday evening (John 20:19-25), celebrated in Jerusalem on Mount Zion.
Recent liturgy offers a special formular for Easter Sunday (different from the Easter vigil). The readings contain the Speech of Peter in front of Cornelius (Acts 10), Seek what is above (Col 3), or Christ our paschal lamb (1 Cor 5). The gospel narrates about the visit Mary Magdalene, the Beloved Disciple, and Peter at the tomb (John 20). There is an additional hymn called Victimae paschal laudes after the second reading.
Traditionally, the celebration of Vespers on Easter Sunday was given a prominent position. It included a procession to the baptismal font where the baptism of the catechumens had taken place during the Easter vigil.
Another particularity of Easter Sunday used to be the anticipation of its liturgy. As the liturgy of the Easter vigil was moving earlier on Saturday, so was the Easter Sunday celebration: first at midnight and eventually even on the evening of Holy Saturday. This resulted in an odd situation when the Easter vigil was anticipated and celebrated on Saturday morning and Easter Sunday was celebrated on Saturday night.
Before the beginning of the Easter celebration, Christians used to exchange the sign of peace. It is since the beginning of the 10th century that they used the Easter acclamation Christ has risen indeed!