Holy days of obligation are the days on which we're expected to go to Mass. This list includes every single Sunday as well a couple special days in the year. This year, Christmas falls on a Monday giving us two holy days of obligation in a row. This means that you must attend two different Masses: a Sunday Mass and then a Christmas Mass. Similarly January 1st, the Solemnity of Mary the Mother of God, is another holy day of obligation and you must again, attend two Masses to fulfill the obligations.