What is the origin and development of the calender of the Western Church? Happy New Year. 

So today, being the first Sunday in Advent, marks the beginning of a new year in the western church, that is the Roman Catholic Church, and those Protestant churches which are descended from it, such as the Lutherans, Anglicans, some in the Reformed tradition, Methodists, &amp;c. How did the calender develop?

I am asking here about the more general "seasonal" calender--Advent, Christmas, Epiphany, Lent, Easter, and Pentecost seasons, not so much about the various days upon which one Saint or another is remembered.
According to a number of sources, liturgical calendars (which is what the 'church year' is more formally known) have been around in some form since the fourth century, and were fully developed (though not in  their present form) in medieval times. The liturgical seasons, and the celebration of saints days, were part of the same process. The Lutheran church and other Protestant churches retained the basic form when their division from the Roman church took place, dropping a large number of saints days but retaining others.

The actual seasons have remained largely unchanged since then, with most of the modifications (in both Protestant and Catholic circles) being to the saint days celebrated.