The Start of the Easter Vigil?

Each of the main sources state the following:

The Roman Missal: The Easter Vigil - "3. The entire celebration of the Easter Vigil takes place at night. It should not begin before nightfall; it should end before daybreak on Sunday." RM, 2nd edition, ICEL translation, 1985

RM, editio typical tertia, still in Latin: 3. Tota celebratio Vigiliae paschalis peragi debet noctu, ita ut vel non incipiatur ante initium noctis, vel finiatur ante diluculum diei dominicae." ."

General Norms for the Liturgical Year and the Calendar, (1969) no. 21 - "The Easter Vigil, during the holy night when Christ rose from the dead, ranks as the 'mother of all vigils.' Keeping watch, the Church awaits Christ's resurrection and celebrates it in the sacraments. Accordingly, the entire celebration of this vigil should take place at night, that is, it should either begin after nightfall or end before the dawn of Sunday."

Paschale Solemnitatis, On Preparing and Celebrating the Paschal Feasts, (1988) no. 78 - "'The entire celebration of the Easter Vigil takes place at night. I should not begin before nightfall; it should end before daybreak on Sunday.' This rule is to be taken according to its strictest sense. Reprehensible are those abuses and practices which have crept in many places in violation of this ruling, whereby the Easter Vigil is celebrated at the time of day that is customary to celebrate anticipated Sunday Masses. Those reasons which have been advanced in some quarters for the anticipation of the Easter Vigil, such as lack of public order, are not put forward in connection with Christmas night nor other gatherings of various kinds."

Most dioceses and their office of liturgy/worship, will publish the official time for nightfall each year to assist in calculating and announcing when the Easter Vigil may begin.