FWIW, at Vigil practice today, we were informed by Father that attending tonight does indeed fulfill the Obligation.
It had been explained to me in an earlier RCIA class that the liturgical day actually begins at 5 pm the evening before. That's why Saturday evening Mass has the liturgy for that coming Sunday, and why Saturday evening Masses don't begin before 5.
Except that on Holy Saturday it is NOT the same Mass as the one celebrated on Easter Sunday....that's the difference.
That is irrelevant. The liturgical day is normally midnight to midnight (so the RCIA class was wrong, it does not start at 5 pm or 4 pm)
Otherwise you would be unable to have a Sunday evening Mass be a Sunday Mass!
1. With a Mass of anticipation, one can use the Sunday liturgical texts generally starting at 4 pm. That is a matter of local legislation, so may vary diocese to diocese.
2. But they can only do this if the Saturday itself is not a higher ranked day. Hence one cannot use Easter Sunday Mass texts on Saturday evening. Nor for that matter could they use the Mass for the Sunday of Advent when December 8th is on a Saturday. You have to use the Immaculate Conception texts. The anticipation of the Sunday liturgy is just that, it is using the Sunday liturgy prior
to the liturgical day and you cannot override the liturgical day of Saturday like that if it is of higher rank
3. However, it doesn't matter what Mass is actually said. Hence a funeral Mass on Saturday evening would count for the Sunday's obligation. Or attending an Eastern liturgy for St. Basil on January 1st fulfills the obligation for Mary Mother of God/Circumcision even though it is not the same liturgical mystery. And for that matter, attending a Greek Catholic Divine Liturgy tomorrow counts, even if it is not an Easter Mass (many Eastern Catholics still observe Julian Easter)
4. When the Easter Vigil was moved to the evening, the legislation in 1955 explicitly allowed it to fulfill the Mass obligation
5. The time which a Mass the day before can fulfill your obligation may be as early as 12pm, and according to most canonists at least as early as 2pm. So a Saturday Mass at 2:30 pm, though not the same texts of Sunday, would count.
6. While in the 1955 legislation one could not receive communion at both Masses (the presumption being with the Easter vigil that the Mass began at about midnight, they were unduly specific about that), current canon law allows communion twice in the same day as long as the 2nd time is in the context of a Mass (technically one could receive a third time, if that third was the viaticum)
Hope that helps
ETA: In older law, pre-1983 at the latest, one needed to attend the liturgy within their own rite on days of obligation in order to fulfill it. Further, previously the 2nd communion not only had to be at a Mass but it had to be a different liturgy, such as attending a nuptial Mass after attending Mass in the morning. This is no longer the case