Is there any official document explaining Sundays being more than 24 hours?
Yes. Quite a bit.
Canon 1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the preceding day satisfies the obligation of participating in the Mass.
I can provide all kinds of references from Church documents at the highest level (Catechism, General Instruction for the Calendar, etc. etc.) but is that really necessary?
Also: (my personal favorite) from Bl (soon Saint!) John Paul II
- 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 [note: that means evening prayer on Saturday]. 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.
vatican.va/holy_father/john_paul_ii/apost_letters/documents/hf_jp-ii_apl_05071998_dies-domini_en.html
Note on the English translation: that sentence (in Latin) reads more like: what is sometimes called the “pre-feastday” (not Vigil as the translator uses) Mass is the “festive Mass” or “Mass of the feast” of Sunday. There’s also a clause which adds “it is the day of the Lord” or “it is Sunday” which the English translator left out completely for some odd reason. Since it’s hard to distinguish “day of the Lord” from “Day of Sunday” in Latin, (they are both “Dies Domini”) it’s difficult to tell which sense he meant—probably both, since it’s really a distinction without a difference.
I can post the original Latin if you need (or just find it on the Latin version of vatican.va)
I would like for you to expound on “there’s more to it than that”.
I didn’t want to bring this up voluntarily. What I meant by that is to say that I personally (and pastorally) feel that if a person were to intentionally manipulate his schedule to attend the Sunday evening Mass (in this particular case, the Mass of the Solemnity on Monday) so as to intentionally miss the actual Mass-texts of Sunday, such a person
would be satisfying the requirement of canon law to attend Mass on either the day-of or the evening-before, but would not truly have kept holy the Lord’s Day of Sunday. The canon would be satisfied, but the true spirit of keeping Sunday would not be satisfied. That’s a pastoral response.