(The following is taken from the U.S. Bishops' Committee on the Liturgy Newsletter vol. 37. This statement is not official law, but it is an official explanation of the law.)
During the past thirty years, the BCL Newsletter has addressed the question of the time for the Easter Vigil on several occasions. Each time, the General Norms for the Liturgical Year and the Roman Calendar, no. 21, has been cited:
"The Easter Vigil, during the holy night when Christ rose from the dead, ranks as the 'mother of all vigils.' (Augustine, Sermon 219: PL 38, 1088). 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."
In 1988, the Congregation for Divine Worship and the Discipline of the Sacraments addressed this question with greater specificity in its Circular Letter Concerning the Preparation and Celebration of the Easter Feasts. After repeating the rubric cited above, the Congregation noted that "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 it is customary to celebrate anticipated Masses (no. 78)."
The intention of the Missale Romanum is clear: the Easter Vigil is to take place in darkness. Thus the approved translation of post initium noctis is after nightfall, that is, after the time in the evening when daylight is last visible. This time is roughly equivalent to astronomical twilight, which is defined by the Naval Observatory as the time after which "the Sun does not contribute to sky illumination." Tables of sunset and astronomical twilight for each locality in the United States are available at the Naval Observatory website.
In Washington, DC, by way of example, sunset will take place at 6:45pm on Holy Saturday, April 15, 2001. However, Astronomical Twilight in the nation's capital will not occur until 8:21pm, or 96 minutes later. Likewise, sunset in Los Angeles occurs at 6:25pm, but Astronomical Twilight (when "the Sun does not contribute to sky illumination") occurs at 7:53pm, about 88 minutes later. While some pastoral flexibility concerning the astronomical mathematics of the question is reasonable, it is clearly the intent of the Church that the Easter Vigil not begin until it is dark.