Why is 5pm Sunday Mass not considered Monday Mass if Saturday PM Mass is Sunday Mass?
A day is midnight to midnight (per canon law and also commonly). For Sunday and solemnities the celebration begins on the prior day’s evening. Also, fulfillment of the Mass obligation for Sunday and solemnities is extended to the prior evening (afternoon) per canon law. That was done to allow for flexibility with various work schedules. The liturgy used in fulfillment does not have to be that of the Sunday or solemnity.
We get a longer Sunday. We do Evening Prayer I on Saturday evening and Evening Prayer II on Sunday. (there is no "Saturday " evening prayer)We do this for Solemnities as well.
I think it is practical to for someone who works weekends that it gives more chances for Mass.
If the succeeding Monday were a holy day of obligation, a 5pm Sunday Mass would satisfy one’s obligation for the following day!
A Saturday PM is still a Saturday PM Mass, as it occurs on a Saturday.
But out of the Mercy of the Church, allows such Mass on the vigil of a Sunday or Holy Day fulfills the Obligation for the following Sunday or Holy Day.
This does cause a lot of confusion.
Only solemnities begin on the evening before.
Sunday begins on Saturday evening, at the time of First Vespers and continues until midnight early Monday. This is because the Church is following the biblical understanding of a day which begins in the evening, when the previous day ends. This is still the case for the Eastern Churches.
Non-solemnities still begin at midnight (so that means most days of the year). Monday begins later because the Church wants to give us as much time as possible to fulfill the Sunday (or other solemnity) obligation.
Saint John Paul II explains this in his encyclical Dies Domini
49. 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 note: prior to 1983, all Masses had to begin before Noon, with some exceptions] 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 [pre-feast] 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.
*The English translator, for some reason, removed the next part of that sentence from the original Latin “dominici diei est” which could mean either “it is the day of the Lord” or “it is the day of Sunday” (they mean the same thing in Latin).