Today is one of the few Holy Days of Obligation left in the Church. They serve as a reminder to us that going to mass, worshiping God in Church with the rest of the community, is not something we do simply when we feel like. It is a duty that we owe to God and to the Church of which we are a part.
God is Lord and we are the servants, the children. The first commandment is to Love God, but if we express that love only when it is convenient, when we feel like it, what kind of love is that?Holy Days of obligation are and should be inconvenient. True love expresses itself most in the inconvenient moments of life.
And yes, it is still considered a grave sin if you simply blow off going to Church today. In the first reading today we have the story of original sin. If we look beyond the symbols of tree and fruit we see that heart of the original sin was Adam and Eve deciding that they could decide for themselves what was right or wrong. That they answered to no higher authority.
This is still our greatest temptation,to think that like petulant children,"no one can tell me what to do." In this season of advent let us be mature adults, and show the virtues of humility, obedience, and gratitude.
Acknowledge your obligation, go to mass, and give God the worship he is due.