While a day like Christmas is fixed in our minds and on the calendars on December 25, many of the important feasts of the Church year move, based upon the date that Easter is set. Easter changes each year moving to the Sunday after the "Paschal Full Moon," and can fall between March 22 and April 25.
In ancient times before calendars were common, most people did not know the dates for the upcoming Liturgical year. On Epiphany Sunday, the upcoming dates were "proclaimed" after the gospel in this way:
Dear brothers and sisters, the glory of the Lord has shone upon us, and shall ever be manifest among us, until the day of his return.
Through the rhythms of times and seasons let us celebrate the mysteries of salvation.
Let us recall the year's culmination, the Easter Triduum of the Lord: his last supper, his crucifixion, his burial, and his rising celebrated between the evening of April 9 and the evening of April 11, Easter Sunday being on April 12.
Each Easter -- as on each Sunday -- the Holy Church makes present the great and saving deed by which Christ has for ever conquered sin and death. From Easter are reckoned all the days we keep holy.
Ash Wednesday, the beginning of Lent, will occur on February 25.
The Ascension of the Lord will be commemorated on May 21.
Pentecost, joyful conclusion of the season of Easter, will be celebrated on May 31.
And, this year the First Sunday of Advent will be on November 29.
Likewise the pilgrim Church proclaims the passover of Christian the feasts of the holy Mother of God, in the feasts of the Apostles and Saints, and in the commemoration of the faithful departed.
To Jesus Christ, who was, who is, and who is to come, Lord of time and history, be endless praise, for ever and ever.
(Taken from the website of Collaborative Ministry)