[quote="TypesAndShadows, post:2, topic:296344"]
The second point is that Mass on a Saturday only counts as Sunday Mass when it is a "vigil Mass" for the Sunday. This is different from the Mass that is specially for that Saturday: it has the prayers and readings of the Sunday Mass*. It isn't to do with watching the clock, although I don't expect a vigil Mass would be allowed early in the day. If you look on a church noticeboard or bulletin, Mass on a Saturday evening will usually have "vigil Mass" or "for Sunday" written next to the time, to let people know that this is the vigil Mass for Sunday rather than the usual Saturday Mass.
*Some vigil Masses have special prayers and/or readings just for the vigil, which are different from the prayers and/or readings on Sunday morning, as well as different from the normal Saturday Mass prayers and/or readings. But they still count as Mass for the feast they're a vigil for. Don't worry to much about this, if it confuses you.
Actually, the Saturday evening Mass IS a Sunday Mass. It isn't a vigil in the regular meaning of the word.
If I understand correctly, Sunday is a privileged day. It begins at First Vespers (sundown) on Saturday and ends at 11:59 on Sunday. That is how Sunday is set up in Liturgy of the Hours. Hence, a Saturday evening Mass at 5 p.m., a Sunday morning Mass at 7 a.m., a 8 p.m. Sunday Mass all satisfy the obligation.
So to recap: Saturday evening Mass can be a Sunday Mass. It has to be in the evening. However, it has to be intended to be a Sunday Mass with the prayers and readings of the Sunday Mass. If however, the priest uses the prayers (collect, offeratory and post-communion) and readings of the Saturday Mass, then it's not a Sunday Mass. So a Mass at 6 p.m. can be either a Saturday Daily Mass or a Sunday Mass. It depends on the readings and prayers and the priest's intention.