Here’s the thing. Yes, the idea of attending Mass in the evening is based upon the Jewish custom, and the early Christian way of reckoning days as beginning at sunset. However, it’s not a matter of strictly following that method, it’s merely based upon it. The Church is taking 2 different methods of determining days and “merging” them.
If you’ll allow me to explain it, it actually is a justification to say that it is an accommodation for those who cannot attend on Sunday morning. The point here is that the Church is being generous and pastoral in making it as easy as possible for people to fulfill the Sunday obligation. In one sense, the Church is saying “we are being generous by allowing the use of both systems (beginning at sunset and beginning at midnight) side by side.” The value of making it possible for more people to attend Sunday mass is more important than the value of choosing one method or the other and being consistent across the week.
Here’s a good paragraph from Bl John Paul II’s Apostolic Letter
Dies Domini
vatican.va/holy_father/john_paul_ii/apost_letters/documents/hf_jp-ii_apl_05071998_dies-domini_en.html
- Because the faithful are obliged to attend Mass unless there is a grave impediment, Pastors have the corresponding duty to offer to everyone the real possibility of fulfilling the precept. The provisions of Church law move in this direction, as for example in the faculty granted to priests, with the prior authorization of the diocesan Bishop, to celebrate more than one Mass on Sundays and holy days, the institution of evening Masses and the provision which allows the obligation to be fulfilled from Saturday evening onwards, starting at the time of First Vespers of Sunday. From a liturgical point of view, in fact, holy days begin with First Vespers. Consequently, the liturgy of what is sometimes called the “Vigil Mass” is in effect the “festive” Mass of Sunday, at which the celebrant is required to preach the homily and recite the Prayer of the Faithful.
To make it easier to read, I’ve removed the footnotes.
Also, note that “vigil Mass” is only in the English translation. The Latin actually says
praefestivae and not “vigil”–the translator’s choice of “vigil” causes some confusion. So the Saturday evening Mass is not the “pre-feast” Mass but the Mass of the feast itself (
festiva)
“Propterea Missae liturgia nonnumquam «praefestivae» appellatae, quae vero reapse pleno iure «festiva» est, dominici diei est,…”
vatican.va/holy_father/john_paul_ii/apost_letters/documents/hf_jp-ii_apl_05071998_dies-domini_lt.html