One of the beautiful things about being Lutheran is that we keep a liturgical calendar. While adiaphora (things Scripture neither forbids nor commands), keeping the liturgical calendar is of extreme benefit to the people of God. Odo Casel (1886-1948), Benedictine monk and pioneer of the Liturgical Movement in the 19th and 20th centuries, reminds us of the liturgical calendar’s importance:
The liturgical year, in celebrating acts, achievements, and historical events, is not concerned with them as such but rather with the eternal content which they gather up and expound. This content is the great work of God for the human race, the redemption by Christ who seeks to rescue man from the narrow limits of time in order to bring him into boundless eternity… The liturgical year, in imitating and to some extent reconstituting the whole development of the Mystery of Christ, is not intended to enfold us in our progressive ascent to God… In practice: we celebrate the season of Advent, not by putting ourselves in the position of unredeemed humanity, but in holding the certainty that the Messiah has already come; we prepare our souls to receive him and we seek the example of the righteous men of the Old Testament, their piety, their perception and their hope; in them we seek the best model to imitate in order to produce in ourselves the desired dispositions.
We do not spend the holy days of Lent as men whom the blood of Christ has not yet bought and justified, but rather as bearing in our souls the seal of the cross and striving to become increasingly conformed to the Lord’s death, so that the resurrection may increasingly manifest itself in us. Even when we are travelling the sorrowful way of the cross with our Lord we always bear in mind the glorious Kyrios [Lord] to whom we cry: “Thou who art seated at the right hand of the Father, have mercy on us!” …[The liturgical year] is the mysterious renewal and application to the faithful of the redemption culminating in the sacrifice of the cross; and of the transfiguration of the Church resulting from the divine resurrection.quoted in Schumacher, 90-91
In short, the liturgical calendar is not the practice of us ascending to God but God condescending to us. (This is the whole point of the incarnation, after all.) Hence Gottesdienst, that is, Divine Service—that liturgy and worship is about God coming to serve us in His Word and Sacraments as we respond with thanks and praise.
Thus, in Advent, we celebrate that Christ has already come as we look forward to His Second Advent when He returns in glory, just like God’s people in the Old Testament. Their faith, therefore, becomes a model for us to imitate as we await His Second Advent. During the Christmas season, for 12 days we celebrate that God became human “to save His people from their sins” (Matthew 1:21). During Epiphany, we celebrate Christ’s revealing Himself to the Gentiles as manifested in the Magis’ visitation as well as His Baptism in the Jordan. During Lent, we practice repentance—even through fasting, trusting in Christ our King’s mercy to forgive us our sins when we cry out, “Lord, have mercy.” During Easter, we lift high songs of praise that Christ is risen from the dead, rejoicing that we shall experience a resurrection just like His (Romans 6:5). And during Pentecost, we recognise Christ’s continuing work in the Church through His Holy Spirit, which began at Pentecost and continues until His imminent Parousia.
Schumacher, Frederick J., and Dorothy A. Zelenko. For All the Saints: A Prayer Book For and By the Church. Volume I, Year 1: Advent to the Day of Pentecost. Delhi, NY: The American Lutheran Publicity Bureau, 2003.