Today is the Solemnity of Mary, the Mother of God. What is a Solemnity and how is it different from a Feast or a Holy Day of Obligation?
Feast Days are days which are celebrated in commemoration of the sacred mysteries and in memory of the Virgin Mary, the Apostles, martyrs, and saints. In earlier times they were Holy Days (holi-days) when people abstained from work, business were closed and were observed by special services.
A Solemnity is a principle holy day celebrating events in the life of Jesus, Mary or the saints. In earlier times they were Major Feast Days, which were not just days of rest but full fledged celebrations.
Liturgically the observance of Solemnities begins on the evening of the day prior to the holy day. Fridays which fall on solemnities are not days of fast or abstinence and no substitute penitential act is required. So when saying the Liturgy of the Hours on Solemnities there are two Evening prayers. Evening Prayer I is for the day before and Evening Prayer II is for the evening of the Solemnity itself, just as there are for all Sunday Hours.
Memorials are lesser feast celbrated in honor of a saint. Among other differences, the Gloria is not said or sung at a Memorial, but is at a Feast, and the Creed is added on Solemnities.
Some Solemnities are also Holy Day of Obligation. In the United States today is such a day. However whenever January 1 falls on a Saturday or a Monday, as it does this year, the precept to attend Mass is abrogated, provided you attended Mass as required yesterday, on Subday.