My Yoke Is Easy, My Burden Light.

From time to time I have been invited to weddings on Sundays – typically in the early afternoon – at which a Nuptial Mass is celebrated. All Catholics are obliged to attend Mass on Sundays but, by a special rule, one can fulfil one's obligation by attending Mass on Saturday evening. The propers of that Mass (readings, changeable prayers etc.) are almost always the same as those of the Sunday, even though it is on Saturday evening. Mutatis mutandis this applies for those Feasts designated "Holy Days of Obligation" ("Days of Precept" in the old terminology) which happen to fall on a weekday. In Australia such occasions are limited to Christmas Day and the Solemnity of the Assumption on August 15th.

I had always assumed that, to fulfil an obligation to hear Mass on a given day, Catholics must hear the Mass of that day. Wedding Masses have their own prayers and readings, therefore a Wedding Mass on a Sunday would not fulfil my Sunday obligation, and so I would still have to attend Mass elsewhere on Sunday morning or Saturday evening. With the children, and dressing for the wedding, and so on, this can be quite tough.

It turns out my assumption is false. Edward Peters (whom I mentioned the other day) explains things.
…a few folks who correctly remind others that there are two attendance obligations coming up seem also to assert that the type of Mass attended determines which attendance obligation can be satisfied thereat, as in, for example, a Mass of Anticipation for the Second of Advent, celebrated at 5 pm next Saturday, can only be applied toward one’s Sunday obligation, not toward Immaculate Conception. That’s an error arising from confusing the canonical obligation on people to attend Mass with the liturgical obligation on priests to celebrate the Mass called for by the rubrics. The people’s canonical obligation to attend Mass is satisfied by their “assisting at a Mass celebrated anywhere in a Catholic rite on the [day required] or in the evening of the preceding day…” (c. 1248 § 2). The law says nothing about what type of Mass is celebrated, only, that it must be a Mass in a Catholic rite.
A Canon Lawyer on the Catholic Answers forum came to a different conclusion. Peters responds to his arguments (references at the link).
The Church imposes an obligation to attend Mass-on-Sunday (a phrasing I prefer because so many people take the requirement “to go to Sunday Mass” to mean “to go to Mass as celebrated on Sunday with the Sunday readings etc.”, which is to prejudice the very point in question) and recognizes the 24-hour period known as Sunday as being available for one to fulfill that obligation. In that respect, then, not only have we an obligation to attend Mass-on-Sunday, but we have a right to fulfill that obligation within a set 24-hour period. Now, just very recently in Church history, the Church has offered us the option of fulfilling our Mass-on-Sunday obligation during some hours on Saturday. We now have extra time in which to fulfill an obligation, but—and here’s the key—having the option of satisfying one’s Mass-on-Sunday obligation on a Saturday in no way deprives us of the right to fulfill our Mass-on-Sunday obligation anytime during the 24 hours of Sunday. Else, the granting of an option with one hand would be to deprive us of a right with the other. Canon law does not work that way.