It is a mortal sin to
disobey your Bishop - and has been since the time of St. Ignatius of Antioch (died in 107 AD). So, yes, if your Bishop has said that you shall go to Mass on December 8th, then it would be a mortal sin to disobey him by not going.
I am very sure that it is not arbitrary; I think it has something to do with Mary being the Patron Saint of the United States, and December 8th is your national feast day. You would be showing love to your Patron Saint and to your fellow countrymen by attending that Mass.

I am also sure that if it would seriously harm you in some way to attend, that you would be given a dispensation by your parish priest.
Ours is on March 19th, for St. Joseph, although the Bishops haven’t declared it to be a Holy Day of Obligation. I’m not sure why not.