This is a good question.  The answer is to be found in Eucharisticum Mysterium #28:

28. Anticipating the Sunday and Feast Day Masses on the Previous Evening
Where permission has been granted by the Apostolic See to fulfill the Sunday obligation on the preceding Saturday evening, pastors should explain the meaning of this permission carefully to the faithful and should ensure that the significance of Sunday is not thereby obscured. The purpose of this concession is in fact to enable the Christians of today to celebrate more easily the day of the resurrection of the Lord.

All concessions and contrary customs notwithstanding, when celebrated on Saturday this Mass may be celebrated only in the evening, at times determined by the local Ordinary.

In these cases the Mass celebrated is that assigned in the calendar to Sunday, the homily and the prayer of the faithful are not to be omitted.

What has been said above is equally valid for the Mass on holy days of obligation which for the same reason has been transferred to the preceding evening.

The Mass celebrated on the evening before Pentecost Sunday is the present Mass of the Vigil, with the Creed. Likewise, the Mass celebrated on the evening of Christmas Eve is the Mass of the Vigil but with white vestments, the Alleluia and the Preface of the Nativity, as on the feast. Nevertheless it is not permitted to celebrate the Vigil Mass of Easter Sunday before dusk on Holy Saturday, certainly not before sunset. This Mass is always that of the Easter Vigil, which, by reason of its special significance in the liturgical year and in the whole Christian life, must be celebrated with the liturgical rites laid down for the Vigil on this holy night.

The faithful who begin to celebrate the Sunday or holy day of obligation on the preceding evening may go to Communion at that Mass even if they have already received Communion in the morning. Those who "have received Communion during the Mass of the Easter Vigil, or during the Mass of the Lord's Nativity, may receive Communion again at the second Easter Mass and at one of the Masses on Christmas Day."80 Likewise "the faithful who go to Communion at the Mass of Chrism on Holy Thursday may again receive Communion at the evening Mass of the same day," in accordance with the Instruction Tres abhinc annos of May 4, 1967, no. 14.

      So, as you can read, the basic answer is "because permission has been granted by the Apostolic See (ie, the Bishop)."  Now, why would bishops grant this permission?  Well, it all goes back to another document, from Vatican II: Sacrosanctum Concillium, which was all about reforming the liturgy.  In response to that reform, the vigil service for Sunday masses (and holy days of obligation) was allowed.  I don't know the exact reasoning, but I do believe it was to encourage more of the faithful to celebrate the eucharist each weekend, as is our obligation.  The idea that Sunday could "begin" after sundown on Saturday night goes back to the Jewish belief in the Sabbath (Saturday) beginning on sundown of Friday.

      You will note that this is only for Sundays and Holy Days of Obligation.  I would also note that big feast days (like Assumption, Immaculate Conception) and holy days particular to religious orders (St. Dominic for Dominicans, Francis for Franciscans, Our Lady of Mt. Carmel for Carmelites, Ignatius for Jesuits, etc) also start their celebrations of those days with the divine office the night before (Evening Prayer I, with Evening Prayer II on the night of the feast proper).

       So, the mass on Saturday is a mass the bishop allows for the vigil of Sunday.  Now, unless Monday were a holy day of obligation, then any Sunday evening mass would have to be a Sunday mass.  I also point out that the mass "propers" (readings and prayers) are different for the Saturday daily mass and the Sunday mass.  So, not every mass on Saturday night need be a Sunday Vigil mass.  It could be the Saturday daily mass or a ritual mass (wedding or funeral) or a votive mass (mass for Mary, Holy Spirit, Sacred Heart).

      In fact, there is nothing stopping your LifeTEEN ministry from having its lifeTEEN mass on a Saturday night as well (if the bishop allows vigil masses).  But, on Sunday, there is no choice.  And, again, the allowing of vigil masses for Sundays and other holy days of obligation belongs to the bishop; and from what I understand, this option has been exercised by most (perhaps all?) bishops since the reform of the liturgy in order to promote greater numbers of laity participating in the mass.

I do hope this helps.

Grace and Peace,
Fr. Chris

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