I attended an Extraordinary Form Mass this morning [Sunday] so I could stay home with a sick child while my husband and older children went to a later Novus Ordo Mass at our parish. I forgot until I was almost there that Mass would not be said for the Epiphany. Should I go to an Extraordinary Form Mass tomorrow for Epiphany? The priest said we should do our best to attend the Epiphany Mass tomorrow, so I do not think I am obligated to. It is possible for me to go, but it would be inconvenient for the family, especially since my son is sick. Thank you, Father.
If your son is sick and needs your care, you do not have an obligation to fulfill, Sunday or not. Your obligation is to your son.
However, you fulfill your Mass obligations according to what is laid down in the 1983 Code of Canon Law. The Code says that you fulfill your obligation by attending Mass on the day of obligation itself or on its vigil. If you went to Mass on Sunday you fulfilled your obligation regardless of what Mass formulary was used.
Also, Epiphany, 6 January, which this year falls on a Monday, is not a Holy Day of Obligation in these USA. (All Sundays are, of course.)
If you are in England, Epiphany has been reinstated as a Holy Day of Obligation. When Epiphany falls on Saturday or Monday, in the Novus Ordo calendar it is transferred to the Sunday (which is always of obligation). If Epiphany falls not on Monday or Saturday, in England Epiphany will be of obligation.
Again, if you are impeded from attending Mass because of sickness, or sickness in the family, or bad weather, or invasion by aliens, forest fires, sneak attacks by Cylons, floods and the like, you do not have an obligation to fulfill.