holy days of obligation

JWorld SG

in the Roman Catholic Church, religious feast days on which Catholics must attend mass and refrain from unnecessary work. Although all Sundays are sanctified in this way, the term holy days usually refers to other feasts that must be observed in the same manner as Sunday.

JWorld SG

In the past, people get day of rest on these holy days.
(That was how the word "holiday" came about).

Cool

JWorld SG

Holy Day of Obligation
In the Catholic Church, the Holy Days of Obligation are the days, other than Sundays, on which the faithful are required to attend Mass. The 1983 Code of Canon Law standardized ten days to be observed as Holy Days of Obligation throughout the Church. These days are:

The Immaculate Conception (December 8 )
Christmas (Feast of the Nativity) (December 25)
Mary, the Mother of God (January 1)
Epiphany (January 6)
St. Joseph (March 19)
St. Peter and St. Paul (June 29)
The Assumption of Mary (August 15)
All Saints (November 1)
The Ascension (39 days after Easter; 40th day of Eastertide)
Corpus Christi (60 days after Easter)
Red vestments are worn at Mass held on the Feast of SS Peter and Paul. White vestments are worn at Mass held on all other holy days of obligation, even those which fall during a season of the liturgical year when some other colour is used.

Observation by country
The conference of bishops in a particular country or region may suppress the observance of these days, or transfer it to another day, with the permission of the Vatican. In some cases the obligation is observed during the following weekend mass. At times if a Holy Day is on a Monday or Saturday the observance will be held during regular weekend mass. They may also make the feast day of county's patron saint a holy day of obligation.

Australia

Belgium

Canada

England and Wales

Ireland

The Netherlands

Poland

United States
By Decree of the National Conference of Catholic Bishops (now the United States Conference of Catholic Bishops), promulgated November 17, 1992 by the Most Reverend Daniel E. Pilarczyk, Archbishop of Cincinnati, and confirmed by the Apostolic See on July 4, 1992, the Holy Days of Obligation for Latin Rite Catholics in the United States are:

JWorld SG

Since we're at it, let me cover the Eastern Orthodox's part:

Eastern Orthodox Church
In a few, predominantly Eastern Orthodox, nations, religious holidays are celebrated on the corresponding day in the Julian Calendar. From 1900 until 2100, there is a thirteen-day difference between the Julian and the Gregorian Calendar, which is used in most of the world as well as in Eastern Orthodox countries for civil purposes. Thus, for example, Christmas is celebrated on January 6 in these countries. The computation of the day of Easter is, however, completely different between the two calendars and does not differ in any straightforward way.

The Liturgical year in the Eastern Orthodox Church is characterized by alternating fasts and feasts, and is in many ways similar to the Roman Catholic year described above. However it is traditionally held to begin on September 1, not on the first Sunday of Advent. It includes the 12 Great Feasts, plus Pascha (Easter) itself, the Feast of Feasts. These feasts generally mark various significant events in the lives of Jesus Christ and of the Virgin Mary. Winter Lent is one name for the extended fast leading up to the Feast of the Nativity of Jesus Christ (Christmas). Great Lent is the extended fast leading up to Holy Week and Pascha. Other times are especially set aside as well. Two other extended fasts are the Apostles' Fast, generally about one to two weeks leading up to the Feast of Saints Peter and Paul, and the fast leading up to the Dormition of Mary, which is for the two weeks prior to that feast, from August 1 to August 14.

The twelve Great Feasts
The Nativity of Mary (September 8 )

The Elevation of the Cross (September 14)

Entrance of Mary into the Temple (November 21)

Nativity of Our Lord and Saviour Jesus Christ (December 25)

Theophany (January 6)

Presentation of Our Lord in the Temple (February 2)

Annunciation of Mary (March 25)

Entry into Jerusalem (Sunday before Pascha)

Ascension (40 days after Pascha)

Pentecost (50 days after Pascha)

Transfiguration of Our Lord (August 6)

Dormition of Mary (August 15)