That's why you can go to Mass on Saturday evening, but it counts for Sunday - the Saturday evening Mass is technically supposed to be after sundown, so that Sunday has already begun, liturgically and Scripturally speaking. Now, this poses a problem for modern man. After all, sundown varies from season to season, but people have a hard time scheduling an event whose start time might vary literally day to day. What to do? The Church allows the local bishop to unilaterally decree a set time for "liturgical sundown" that holds throughout the year for when Sunday liturgy may begin. So, Saturday evening Mass can only start after 4:30 PM or 5:00 PM, or whatever time the bishop has set. Anything prior to that only counts for Saturday, not Sunday. Anything prior to that uses Saturday's Mass readings, not Sunday's.
And this is precisely how we calculate that Jesus' body spent three days in the tomb:
- 1st day: Jesus died on Friday before sundown, so He is dead on Friday. Nicodemus has to get His corpse into the tomb before sundown in order to honor the Sabbath rest. We assume that he, being a good Jew, managed this. Mosaic law decreed that anyone hung on a tree should not hang overnight, but should be buried before sundown (Deut 21:23).
- 2nd day: Friday sundown to Saturday sundown.
- 3rd day: Once the sun is down on Saturday evening, the third day has already begun. Jesus rises some time after Saturday sundown, possibly around midnight or 3 AM, so He rises on the 3rd day.
This also explains Easter Vigil Mass. Easter Vigil Mass is any Mass that begins after Holy Saturday sundown. As long as it begins after Saturday sundown, it begins on the third day. In fact, the rubrics indicate that the Easter Vigil Mass is not to begin until the first star can be seen in the sky.
Incidentally, there are actually three different Easter Masses: the Easter Vigil Mass, the Mass at sunrise and the Mass for the day. Each Mass has different readings and a different significance in the life of the Church. But it is the Easter Vigil Mass that all Catholics are encouraged to attend.
Only the Easter Vigil Mass has not one, but two different plenary indulgences attached: the renewal of baptismal vows at Vigil Mass is a plenary AND the fact that you attended a Mass wherein someone received First Communion is also a plenary. Now, you can only win one plenary indulgence a day, but the fact that there are two in operation here is kind of neat. The Easter Vigil is considered the Mother of All Feasts - it is the Mass from which all other Masses draw power and grace. It is the conduit of grace into the full liturgical year.
It's also a lot of fun to watch the candidates get baptized, if only because baptismal water is supposed to be running (thus "living" water) and cold ("we are baptized into His death"). The ancient instructions preferred a river to a lake (living water over still water) and cold water to hot. I have been to baptisms where the candidate accused us of having put ice into the water. To which, I don't see the problem, because we took the ice out before the Mass started, so...