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 )
birth of the Virgin Mary to Joachim and Anne
The Elevation of the Cross (September 14)
the rediscovery of the original Cross on which Christ was crucified
Entrance of Mary into the Temple (November 21)
the Virgin Mary's first entry into the Temple around the age of 3
Nativity of Our Lord and Saviour Jesus Christ (December 25)
the birth of Jesus, or Christmas
Theophany (January 6)
the baptism of Jesus Christ, Christ's blessing of the water, and the revealing of Christ as God
Presentation of Our Lord in the Temple (February 2)
Christ's presentation as an infant in the Temple by Mary and Joseph.
Annunciation of Mary (March 25)
Gabriel's announcement to Mary that she will conceive the Christ, and her "Yes"
Entry into Jerusalem (Sunday before Pascha)
known in the West as Palm Sunday.
Ascension (40 days after Pascha)
Christ's ascension into Heaven following his resurrection.
Pentecost (50 days after Pascha)
The Holy Spirit comes and indwells the apostles and other Christian believers.
Transfiguration of Our Lord (August 6)
Christ's Transfiguration as witnessed by Peter, James and John.
Dormition of Mary (August 15)
The falling asleep of the Virgin Mary (cf. the Assumption of Mary in Western Christian)