The Church year is divided into seasons, each representing a specific focus in the worship of the Church.  The Church Year begins with the Season of Advent (during November or December each year) and ends during the Season after Pentecost (also referred to as Ordinary Time).  The seasons of the Church Year, which also generally have corresponding colors (for vestments, altar dressings etc.), are as follows in order:

Advent

Christmas

Epiphany

Ordinary Time (after Epiphany)

Lent

Easter

Pentecost

Ordinary Time (after Penetecost)

Season Summaries:

The Season of Advent
 
Theme: Anticipating the Coming of the Lord

Liturgical Color: Blue or Purple

Dates: Advent begins on the fourth Sunday before Christmas and ends on the day before Christmas.

The Season of Christmas
 
Theme: Celebrating the Incarnation of God in Jesus Christ

Liturgical Color: White

Dates: Christmas Season begins at sundown on 24 December and ends at sundown on 5 January.

The Feast of the Epiphany

Theme: Jesus Manifests Himself as God
 
Liturgical Color: White

Dates: Epiphany falls on 6 January.

*Ordinary time (after Epiphany) follows the Feast of the Epiphany and lasts until the beginning of Lent.  The liturgical color during this time is green.

The Season of Lent
 
Theme: Retreating Into the Wilderness with Jesus
 
Liturgical Color: Purple

Dates: Lent is a forty-day period before Easter.  It begins on Ash Wednesday and ends on Holy Saturday (the day before Easter).  We skip Sundays when we count the forty days, because Sundays commemorate the Resurrection, and thus the period in which we celebrate Lent is actually longer than 40 calendar days.

The Season of Easter

Theme: Celebrating the Resurrection of the Lord

Liturgical Color: White
 
Dates: Easter Season begins on Easter Day and lasts 50 days, ending on Pentecost.

The Feast of Pentecost

The Feast of Pentecost falls on the fiftieth day after Easter.  Pentecost is celebrated by Christians as the birthday of the Church by the power of the Holy Spirit.  The color for Pentecost Sunday is red.

The Season After Pentecost (also called "Ordinary Time")

Theme: The Church fulfills the Great Commission

Liturgical Color: Green
 
Dates: The Season After Pentecost lasts from the day after Pentecost to the day before Advent.
 

*See also:

Church Year Season Summaries

Liturgical Colors

Church Year Graphic