Can you explain reasons for and against observing Lent for Reformed Christians? Do any Reformed denominations observe Lent? I've noticed that it is not only a Roman Catholic practice, but many mainline denominations observe Lent.
This is a longstanding historical and theological issue. By the time of the Reformation, the Roman Catholic Church had instituted a great number of practices that lacked any biblical warrant or precedent. Many of them became established, even conscience-binding practices. As the Reformers began to plea for a purification of the church, the issue of authority became central. The question was: does the church have the authority to bind the conscience of the believer beyond what Scripture indicates? The Roman Catholic Church said yes, and the Reformers essentially said no. Some Reformers took a really strong stance on this. In time, the Reformation would give birth to different denominational branches, i.e., Lutheran, Anglican, and Reformed. In the area of worship, the former two have adopted a more conciliatory approach, and will allow the church to practice things (whether in worship or holidays), as long as the Bible does not explicitly or implicitly forbid them. This is often called the "normative principal" of worship. Those inheriting a Reformed theology (which would include the OPC) have adopted the stance that the church is only to practice in worship what the Bible actually establishes, often called the "regulative principle of worship." Many in the Reformed tradition would exclude the practice of Lent on this basis—it lacks scriptural warrant.
Furthermore, the Bible's liturgical calendar is remarkably simple—it is the Lord's Day. While the Old Testament had a very complex system of days, all foreshadowing the redemptive work of Christ to come, the New Testament celebrates the accomplishment of that event with profound clarity and simplicity (Heb. 3-4). Our Confession echoes an appreciation for the simplicity of New Covenant worship regarding the sacraments (WCF 7:6), worship (WCF 21:1), and the Sabbath day (WCF 21:8-9). I believe this concern for a biblical simplicity is why we don't follow the liturgical calendar of Roman Catholic Church. The conscience is a frail thing, and only God has the right to exercise lordship over it.