Liturgically, the day begins at sundown and both the Holy Week and Paschal services are moved to the beginning of the Liturgical day “in anticipation”. So the Lamentation service of Holy Saturday will have been said on the calendar-Friday and the first services of Pascha, the Rush Procession and Orthros (Matins), will start on calendar-Saturday. So, Pascha begins on Saturday, but also on Sunday depending on your reference point.