Holy day of obligation in the context of Remember the sabbath day, to keep it holy

Church attendance is a central religious practice for many Christians; some Christian denominations require church attendance on the Lord's Day (Sunday).

In addition to being based upon the spirit of the Ten Commandments ("Remember the Sabbath day, to keep it holy"), the importance of church attendance in Christian theology is delineated in Hebrews 10:25, which implores the believers: "Let us not neglect our church meetings, as some people do, but encourage and warn each other, especially now that the day of his coming back again is drawing near." Certain early Christian saints emphasized the necessity of church attendance for the continuance of the faith, such as John Climacus, who declared that "a sure sign of the deadening of the soul is the avoidance of church services." The Lutheran theologian Balthasar Münter [da] stated that church attendance is the "foundation for the Christian life" as "the Christian Bible and the sacraments provide the framework for the faith"; he also states that it is important for believers because it aids in the prevention of backsliding, as well as offers "the company of other believers". The Catholic Church teaches that on Sundays and other holy days of obligation, the faithful are required to attend Mass. The Westminster Confession of Faith held by many Reformed Churches teaches first-day (Sunday) Sabbatarianism and the duty of church attendance on this day. Similarly, Methodist Church also requires attending "the public worship of God".

Principal Feasts are a type of observance in some churches of the Anglican Communion, including the Church of England, the Episcopal Church (United States), and the Anglican Church of Canada. All Principal Feasts are also Principal Holy Days, sharing equal status with those Principal Holy Days which are not Principal Feasts. They are considered to be the most significant type of observance, the others being Festivals, Lesser Festivals, and Commemorations. As with all Principal Holy Days, their observance is obligatory. The Anglican Principal Feasts and Principal Holy Days are somewhat comparable to Roman Catholic Solemnities and Holy days of obligation.

Printed calendars, in addition to its main function to display dates and related information, usually in a table format, also often contain additional information relevant for specific groups – for instance, a Christian liturgical calendar will show holy days and liturgical colours, while a calendar for amateur astronomers will highlight phases of the moon, conjunctions and eclipses. Calendars for general public may also contain various interesting facts (e.g., associated with anniversaries of important people or important events). Various facts were also printed on the back sides of the sheets of a tear-off calendar. In modern times, calendars may also have a decorative purpose.