Michael Spencer has a post up that encapsulates most of why I like using the liturgical calendar as a basis for worship and Christian living.
I would argue (in addition to Michael's points) that how we celebrate time deeply shapes us as people, and increasingly, American holidays are shaping us as sentimental consumers, and the Christian year shapes us around the person and work of Jesus Christ. Case in point, Spencer points out that no church he was part of when he was younger ever chose Pentecost over Mother's Day. To put it baldly, we exalt the nuclear family over the Holy Spirit's work in the church, and part of the reason is that, for most evangelical Christians, Mother's Day is an obligatory holiday and Pentecost feels optional.
Here's a great quote that starts the post:
traditions. Instead, we have these fads that are built
on the strengths and talents of individual leaders. … But a real
tradition can be handed on to anyone, from generation to generation.
It’s hard to hand these evangelical fads down like that, so it seems
like we’re always starting over. It’s hard to build something that
really lasts." – Joe Carter
Read Michael's post for more good (and practical) thoughts on using the lectionary and Christian year as a basis for local church worship.