The standard time is 4pm, but it depends in part on the jurisdiction. Some dioceses allow a Saturday “vigil” Mass to be done earlier than the standard 4pm. IIRC, Las Vegas is one such, and there may be others. In some cases it’s allowed on a regular basis but in others an ad-hoc basis for a particular occasion. If there’s any question, a quick call to the rectory or the Chancery should give the specific answer.
Canon Law says “vespere” which is 4:00 PM on a modern clock. Vespere is the Latin for evening. Vigil means to stay awake at night (think "the shepherds kept vigil).
Can. 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.
Can. 1248 — § 1. Praecepto de Missa participanda satisfacit qui Missae assistit ubicumque celebratur ritu catholico vel ipso die festo vel vespere diei praecedentis.
A vigil, properly speaking is “in the night.” If it’s before sunset, then it’s not actually a vigil (although that word is often used in English), but as long as it’s after 4:00 it is an evening Mass, and it fulfills the obligation for the next day.
I have seen in two different diocese in Florida that churches had a 3:30 Saturday Mass (Both places had a late one about 7 as well. Though in one of the churches there was only one Satuday evening Mass in the summer. Here it seems the earliest I see is 4:30. This is something I would trust the local church in. (If it is a case if a wedding Mass “counts” then I would ask the diocese)
This terminology can get confusing. For example, Christmas Eve is considered the entire day by many. And Easter vigil used to have its own rite distinct from the Easter Mass. I believe Fr. David nailed it by citing the Latin “vespere” which I believe more reflects the Litany of the Hours and by not using “evening” which could mean almost anything in the English.
I’d imagine 5PM. Here’s why: 99% of weddings, especially Catholic weddings, take place on a Saturday afternoon. It gives the church staff, especially if there’s been a full Nuptial Mass, to clean up and set up the church for the next Mass.
1988’s Paschale Solemnitatis is clear on this point
“The entire celebration of the Easter Vigil takes place at night. It should not begin before nightfall; it should end before daybreak on Sunday.” This rule is to be taken according to its strictest sense. Reprehensible are those abuses and practices which have crept into many places in violation of this ruling, whereby the Easter Vigil is celebrated at the time of day that it is customary to celebrate anticipated Sunday Masses.
Those reasons which have been advanced in some quarters for the anticipation of the Easter Vigil, such as lack of public order, are not put forward in connection with Christmas night, nor other gatherings of various kinds.
The Passover Vigil, in which the Hebrews kept watch for the Lord’s passover which was to free them from slavery to Pharaoh, is an annual commemoration. It prefigured the true Pasch of Christ that was to come, the night that is of true liberation, in which “destroying the bonds of death, Christ rose as victor from the depths.”
From the very outset the Church has celebrated that annual Pasch, which is the solemnity of solemnities, above all by means of a night vigil. For the resurrection of Christ is the foundation of our faith and hope, and through Baptism and Confirmation we are inserted into the Paschal Mystery of Christ, dying, buried, and raised with him, and with him we shall also reign.
The full meaning of Vigil is a waiting for the coming of the Lord.
The Easter Vigil Mass absolutely must begin after sunset. Phemie just quoted the law on this. When Rome uses the term “Reprehensible are those abuses” it should be clear that such is not to be done.
Before (about) 1970 it was common for the Easter Vigil Mass to begin in the morning of Holy Saturday, because by canon law, all Masses had to begin before Noon. That caused a serious disconnect, to say the least. It just made no sense celebrating an event that happened in the middle of the night, but doing so in the morning hours. Thankfully that was corrected.
In some places (and I know it happened, and still does in the US) some parishes unfortunately begin the Easter Vigil Mass too early. Every year since I became a priest, around Ash Wednesday, we get a circular letter from the bishop telling us in no uncertain terms that the Easter Vigil must begin after sunset. The fact that we keep getting it is a good indication that some pastors still “don’t get it.” It’s frustrating.
Anyway, we have a habit of calling the Saturday evening Mass a “Vigil Mass.” That’s not always correct. A vigil, by definition, must be done during the night. That is what the word means, after all. There’s nothing wrong with a 5 PM Saturday Mass as the Mass of Sunday; in fact, it’s one of my pet peeves to post in support it. Yet, if it’s before sunset, it doesn’t meet the definition of a “vigil Mass,” although it does fulfill the Sunday obligation. Anyway, that’s just the definition of the word. It’s not a topic I choose to make an issue; just clarifying the definition, since that’s the topic here.
The actual time seems to vary by jurisdiction. The Canon Law: Letter & Spirit, Canon Law Society of Great Britain and Ireland, 1995 has:What is “the evening of the previous day”? Despite the view of some commentators that this should be interpreted as beginning only at 1400 hours (2 PM) on that day, it is the firm view of this commentary that the evening of the previous day begins at midday (12 noon) on that day itself. In some dioceses there is a local regulation to the effect that the so-called vigil or anticipated Mass may not be celebrated before, say 5 or 6 PM…
Those regulations do not in any way concern the time prescribed for fulfilling the obligation to assist at Mass: thus e.g. if in such a diocese a person were to attend a nuptial Mass in the early afternoon on Saturday, that person would have fulfilled the obligation … of this canon 
Actually, the “morning Vigil” was before so-called “reforms” of 1955, when the Roman Holy Week rites were severely altered. That was clearly an error that needed to be corrected, and to my mind, the adjustment in timing for the rites of Holy Thursday and the Easter Vigil are just about the only positive thing to have come from that. IIRC, while those “reforms” didn’t absolutely require that the Easter Vigil begin “in nocte” (i.e, after dark), it was required that it not be done before “vespere” and in my experience, parishes routinely did it stating between 5pm and 7pm.
The rule about Masses beginning before noon changed in 1953 (although indults to allow later Masses had been granted during the war). It was at that time that Evening Masses begin to be celebrated, in limited situations. By the time I started school in 1959, evening Mass was common. My classmates and I received our First Communion at an evening Mass in 1960 and were confirmed at one in 1961.
While my mother recalled Lent ending after the morning Mass on Holy Saturday, that had changed by the time I was born in 1953. The Easter Vigil was revised in 1951 and my twin cousins were baptized in a nighttime Vigil in 1953, the first time there had been Baptism at the Vigil in our parish.
The earliest time a Vigil Mass can begin is at whatever time the pastor wishes to schedule it.
The on the ground reality is that definitions, past practices etc. mean nothing.
Our local Vigil Mass begins at 3:30 and dusk did not occur here until after 7:30, more than four hours after Mass began.
It is what it is.
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