Are you trying to prove Sede Vacantists wrong with your stellar refutation of Sede Vacantism by saying "How did you celebrate, liturgically, yesterday’s feast day, “The Chair of St. Peter”?
You got the Sede’s good there.
It shows that when you say “no flame wars please”, you’re obviously being dishonest, and in fact want to start a flame war because you think you’ve got the end-all to the argument.
Even if everybody did celebrate this feast day (which is important nonetheless), why would somebody believing the Chair is Vacant, not be able to celebrate it.
FYI, Sede’s don’t disbelieve in the Papacy as do non-catholics, they just believe that the Church is in a Papal Interregnum, just like the other 200 plus times in Church history it’s happened.