Wednesday, January 1st, we celebrate the Solemnity Of Mary, Mother of God. This is a Holy Day Of Obligation and will be celebrated this year with a 9 AM Mass at St.Peter and a noon Mass at St. Mary's at noon.
Since a "grave cause" is needed to excuse one from this obligation, it would be a serious or mortal sin to willfully skip Mass on Sunday or a Holy Day of Obligation, as the Church teaches. Reasons such as the necessity to work to support one's family, child care, personal sickness or the care of the sick, necessary travel etc. would excuse a person on a particular occasion.