A historically distinctive feature of Reformed worship is the regulative principle: only what Scripture explicitly commands or warrants is permitted in public worship, as opposed to the Lutheran principle that what Scripture does not prohibit is allowed. Applied strictly, this principle led many early Reformed and Puritan churches to refuse the observance of Christmas, Easter, and other church festivals as human inventions without scriptural warrant. Calvin's Geneva and the Scottish and English Puritan movements minimized or abolished the liturgical calendar on these grounds.

In practice, contemporary Reformed churches vary widely. The more strictly confessional bodies (such as the Free Presbyterian Church of Scotland or some continental Reformed churches) maintain historic resistance to Christmas and Easter celebrations in worship. Many mainstream Reformed and Presbyterian churches (such as the Christian Reformed Church and the PCUSA) now observe Advent, Christmas, Lent, and Easter as part of a recovered liturgical calendar. The Westminster Directory for Public Worship (1645) did not prescribe a church year. The variation on this point reflects both the genuine freedom allowed by the Westminster Standards and shifts in cultural practice over centuries.