just a random question that popped into my head today. I know that Good Friday is supposed to be the one day of the year that Mass is not celebrated, but was just wondering if for some reason a priest were to celebrate Mass on that day, would it be invalid, or just illicit? (or maybe 3rd option is there some sort of special exception where it's acceptable)

"Invalid" means 'improper form or improper matter'. Any Mass said, by a legitimately ordained priest, with the proper form and proper matter is valid by definition.

Moreover, if the priest was to use improper form, for example instead of saying 'this is my body' he says 'by the power of Greyskull!', or uses improper matter, for example trying to consecrate beer and pretzels instead of bread and wine, then the fact the Mass was invalid would be so glaringly obvious that no reasonable person would even need to ask whether or not it was valid.

In short, an invalid Mass is not something you are ever likely to encounter in your lifetime.

A full Mass would be deeply illicit though. [There is a provision for saying the prayer of consecration in a back room if one runs out of hosts during the distribution of communion in the Good Friday liturgy, and I suppose the consecration is technically what makes the Mass, but I can't think of any other Mass that would be licit on Good Friday.]

Well, truthfully, since there is no 'Mass', per se, there is also no requirement for there to even be communion at all. If you don't have enough consecrated hosts to have communion, then the priest can just as easily say 'no communion today', there is really no requirement to consecrate more. Sure, people will probably complain if there is no communion, but that is because they are childish, and they think of communion as something they 'get' during Mass, and they don't want to go to church if they aren't going to 'get something.' This attitude of 'getting stuff' is the reason why Ash Wednesday is usually a bigger day than Easter.

I agree (traditionally, the laity didn't receive communion on Good Friday at all). However, I'm merely pointing out that there is currently a provision for that in the rubrics of the Good Friday liturgy.

Proper intention is also required for validity.

A priest can licitly celebrate Mass on Good Friday to consecrate Hosts for Viaticum - or at least that used to be the case!

Oh yes! I've heard of this too!

What propers would he use? (I'm not arguing with the claim, just exploring it.)

In the old rite, a Votive Mass of the Sacred Heart. I don't know about the new rite.

On Holy Saturday, a priest doesn’t also celebrate the Mass.

Am I right?

The Easter Vigil is on Holy Saturday

I figured it would be a votive Mass of something. I don't know that the current GIRM says what to do, nor anyplace else.

BTW, only the Chrism Mass can be celebrated on the morning of Holy Thursday. Parish Masses must be the Mass of the Lord's Supper in the evening.

I don't remember a Vigil Mass before Vat. II. What was done back then?

The Easter Vigil was very early on Saturday morning. Very few people went.

But that would require reading the priest's mind, which the laymen in the pews are in no position to do, and stressing that point tends to encourage the scrupulous to doubt the validity of every Mass they attend. Moreover, a lack of intention, if it is discernable at all, is likely to be discernible in a lack of proper form or matter.

Nevertheless, intention is a necessary part of validity and can't be left out. It is certainly best to assume in the absence of significant evidence to the contrary.

I think it was generally celebrated around 8:00 am and ended with a shortened Vespers around noon, at which point the Lenten fast also ended. But the Easter Vigil's being celebrated in the morning ended in 1955 (in some places, in 1951).