Why should the Church number its days by observing a liturgical calendar each year, moving from Advent, Christmas, Epiphany, Lent, Easter, and Pentecost?
Because the annual church calendar retells the Life of the Lord Jesus Christ. By celebrating the major events of Jesus' life, the Church affirms the historicity of His life instead of dismissing the story of Jesus as an abstract collection of timeless morals.
The gospel of Jesus Christ is rooted in time because it is the account of historical events.
We are not saved from time, but in time-saved from sin and death and sharing His authority so His lordship will be known over all peoples, all places and times.
Just as some households order their years according to birthdays and anniversaries, the Church has historically held it to be a blessing to order the yearly pattern of our own lives according to Jesus' coming, His birth, His manifestation to the nations, His suffering and death, His resurrection, and the outpouring of His Spirit.
Duane Garner's For Signs and Seasons gives a defense for the annual church calendar while also explaining the biblical and historical background for each liturgical season. The book also provides liturgies for each season, including Scripture readings, responses, and prayers.