From USCCB:
Easter liturgies include Easter Vigil, celebrated on Saturday evening. Easter Vigil is the "greatest and most noble of all solemnities and it is to be unique in every single Church" (Missale Romanum, "Rubrics for the Easter Vigil" (EV), no.2). On this holy night, the Church keeps watch, celebrating the resurrection of Christ in the sacraments and awaiting his return in glory. It is the turning point of the Triduum, the Passover of the new covenant, which marks Christ's passage from death to life. Therefore, the Easter Vigil does not correspond to the usual Saturday evening Mass and its character is unique in the cycle of the liturgical year.