I usually attend the extraordinary form of the mass. But I couldn't make mass this Friday, when the epiphany was celebrated in the extraordinary form. Now if I go to the extraordinary form on Sunday, I will, in some way, be going to mass according to the requirements of the holydays in the u.s. as per the epiphany, but I won't actually attend an epiphany mass since the extraordinary mass will not be the epiphany mass. Is there any definitive say from the church on how to handle this? It seems to me that the spirit of the law would be that I should try to get to an epiphany mass, but that by the letter of the law I am really only obliged to attend mass on the day appointed — just like if I went to an eastern rite mass on the holy day. Am I correct?
You are certainly correct regarding the fact that you are not obligated to attend the Ordinary Form of Mass this Sunday in order to hear an Epiphany-themed Mass.
The way the law is written, the obligation is to go to Mass on a particular day (or the evening before), not to hear a particular set of readings or liturgical prayers. The law expressly guarantees the faithful's right to fulfill this obligation by attending Mass in any Catholic rite, even if that rite is not celebrating the same saint or event.
Now, on certain days like Christmas, every Catholic rite lines up with a common celebration, but when it comes to other holydays of obligation, they may differ dramatically in what they are celebrating.
In the United States (1) January 6th was abolished as a holyday of obligation and (2) the liturgical celebration of Epiphany transferred to the Sunday between January 2 and January 8 in the Ordinary Rite. (See here.)
The first part of that applies to all Latin Rite Catholics in the United States, whether they normally attend the Ordinary or the Extraordinary Form.
No legal obligation has been created for Extraordinary Form attendees to do anything special on the Sunday between January 2 and January 8, whether they attended the Extraordinary Form on January 6th or not.
This is equally true of other holydays of obligation that have been abolished or transferred in the United States. There are no special "Extraordinary Form holydays of obligation." There is one set of holydays obligation that bind all Latin Rite Catholics in the United States.
They could change that in the future, but that's the way the law is written now.
So you do need to go to Mass this Sunday, but because it's a Sunday. You are not obligated to go to an Ordinary Form Mass in order to hear an Epiphany-themed service. You are free to go to an Extraordinary Form Mass or a Mass in a non-Latin Catholic rite.
As to whether the spirit of the law suggests going to an Epiphany-themed Mass since you missed the Extraordinary Form celebration on January 6th, I think it depends on what you mean by "the spirit of the law."
Sometimes this phrase is meant to imply that you would be doing some thing wrong (even if allowed according to the wording of the law) by violating the law's intent.
If this is what is meant then I don't think you are violating the spirit of the law. If the Church wanted to impose such a requirement it would not allow you to fulfill your obligation to attend on holydays by going to other Catholic rites that may not be celebrating the same thing.
John Paul II knew full well when he approved the relevant canon–canon 1248