The language might be slightly daunting for some (it’s an abstract for a seminary essay), but this post from Halden briefly outlines most of the reasons we are practicing the Christian liturgical calendar during our Sunday evening gatherings.
The point is that the way we practice time powerfully shapes us as a certain kind of people. The Christian liturgical year has historically been a powerful way of forming Christian identity and everyday practice, but this has been "largely lost in the contemporary evangelical church, the life of which has
rather become dominated and determined by the calendar of the
nation-state and market (which tell us, above all, when we must shop)." There are all kinds of liturgical calendars, and the one that most Americans (including evangelical Christians) celebrate and adhere to is the liturgical calendar of the consumer.
So while most evangelical Christians have great intentions for being missional in their everyday lives, we have to face the fact that missional people don’t just fall from trees. They have to be formed. And when our lives are formed by the rhythms of shopping, watching TV, consumption, and paying credit card bills, we find it doesn’t actually make us into missional people, those who can faithfully witness to God’s salvation in Christ. The change has to go deeper and be more radical if we are going to see the church fulfill her call to "implement the achievement of Christ" (Tom Wright’s phrase). We won’t be formed as a missional people unless we radically re-orient our lives around Christ, and away from the liturgies of consumerism.
"Through a recovery of the liturgical year, Christian churches have an
opportunity to reclaim a holistic and missionally oriented ecclesial
self-consciousness which is vital to the faithful witness of the church
in the culture of late-modern capitalism."