Good question.
It’s easier to understand this if you learn how to pray the liturgy of the hours. The prayer times in the liturgy of the hours include Morning Prayer (morning), Evening Prayer (early evening), and Night Prayer (before bed). There are some other prayer times that I omitted.
So, a normal liturgical day goes like this:
8AM Monday: Monday Morning Prayer
6PM Monday: Monday Evening Prayer
10PM Monday: Monday Night Prayer
8AM Tuesday: Tuesday Morning Prayer
6PM Tuesday: Tuesday Evening Prayer
10PM Tuesday: Tuesday Night Prayer
Most liturgical days begin with Morning Prayer in the morning and Night Prayer in the night.
However, solemnities (Sundays and Holy Days of Obligation) begin with the Evening Prayer on the
previous day.
So, the liturgical calendar ends up looking like this on weekends:
8AM Saturday: Saturday Morning Prayer
6PM Saturday: Sunday Evening Prayer I
10PM Saturday: Sunday Night Prayer I
8AM Sunday: Sunday Morning Prayer
6PM Sunday: Sunday Evening Prayer II
10PM Sunday: Sunday Night Prayer II
Yes, the church takes the early beginning of Sunday seriously. So seriously, that anything prayers intended for Saturday night will have the word Sunday next to them, even if they have Saturday’s calendar date! A common newbie mistake is to look for Saturday night’s prayers, and wonder why you can’t find any.
Once you understand this, vigil Masses make perfect sense…when the priest prays evening prayer on Saturday night, liturgically, Sunday has arrived, and it’s impossible to have any further Saturday Masses. At that point, every Mass must use the Sunday liturgy, and is therefore a Sunday Mass which fulfills your obligation.
After you understand the Liturgy of the Hours, common questions like “What happens when Christmas is on a Monday?” become very easy to answer. If a Mass occurs during Sunday’s liturgical day, then the Sunday liturgy is used, and the Mass fulfills your Sunday obligation. If the Mass is late enough to occur on the Christmas liturgical day (which begins on Sunday evening, because Christmas is a solemnity), then the Christmas liturgy is used, and the Mass fulfills your Christmas obligation. Obviously, a single Mass could not fulfill both of these conditions, so you do need to go to Mass two days in a row (or twice on one day) when this situation occurs.
If the Saturday night Vigil Mass is because of the Jewish view of time, then why is it that on Sunday, the obligation can be met anytime from 12 AM to 11.59 PM? Wouldn’t it make sense that the timeframe for the Sunday obligation would end at the same time?
And no, I’m not trying to bash the Saturday Mass - I happen to think that it’s one of the best things that ever happened to the Mass.