Which Day of the Week Is the First Day of the Year AD?

by oaeen
BC and AD

The first day of the year Anno Domini (AD), also known as the Christian era, marks a significant point in history. Determining the exact day of the week for January 1st, AD 1, involves a fascinating interplay of historical, calendrical, and astronomical studies. This article delves into the complexities of early calendars, the historical context of the transition from the Roman calendar to the Julian calendar, and the methods used to determine the day of the week for the first day of AD 1.

The Julian Calendar: Historical Background

To understand the day of the week for January 1st, AD 1, we must first explore the Julian calendar, which was in use at that time. The Julian calendar, introduced by Julius Caesar in 45 BC, was a reform of the Roman calendar. It was designed to more accurately align the calendar year with the solar year.

The Roman Calendar

Before the Julian reform, the Roman calendar was a complex and often inconsistent system. It originally consisted of ten months, with a later addition of January and February. The calendar was primarily lunar, with months based on the phases of the moon. However, this system led to significant discrepancies between the calendar year and the solar year.

Julian Reform

Julius Caesar’s reform aimed to address these discrepancies. The Julian calendar introduced a 365-day year divided into 12 months, with an additional leap day added every four years to account for the approximately 365.25 days in a solar year. This reform established a more consistent and predictable calendar system.

January 1st, AD 1: The Start of the Christian Era

The designation of AD 1 as the beginning of the Christian era was established centuries after the events it marks. The AD (Anno Domini) dating system was introduced by the monk Dionysius Exiguus in the 6th century AD. His goal was to create a calendar system based on the estimated birth year of Jesus Christ.

Dionysius Exiguus and the AD System

Dionysius Exiguus aimed to replace the Diocletian era, which was used at the time, with a calendar based on the birth of Christ. According to his calculations, the year AD 1 followed the year 1 BC, with no year zero in between. This system became widely adopted in Christian Europe and laid the foundation for the modern Gregorian calendar.

Determining the Day of the Week for January 1st, AD 1

To determine the day of the week for January 1st, AD 1, we must consider the Julian calendar system and the methods used to calculate days of the week. This involves understanding the concept of the “day of the week cycle” and the relationship between calendar dates and weekdays.

The Seven-Day Week Cycle

The seven-day week cycle has been used by various cultures throughout history, including the ancient Babylonians, Hebrews, and Romans. The Romans adopted the seven-day week in the 1st century AD, influenced by the Jewish Sabbath. This cycle has continued uninterrupted to the present day, making it possible to trace days of the week backward or forward in time.

Astronomical Calculations

Astronomers and historians use various methods to determine the day of the week for historical dates. One such method involves the Julian Day Number (JDN) system, which assigns a unique number to each day in a continuous sequence starting from January 1, 4713 BC. By calculating the JDN for a given date, we can determine the corresponding day of the week.

Calculating January 1st, AD 1

To find the JDN for January 1st, AD 1, we use the formula for converting a Julian calendar date to a JDN. The formula is as follows:


Plugging these values into the formula gives us the JDN for January 1st, AD 1. We can then determine the day of the week by taking the JDN modulo 7. The result corresponds to a day of the week, with 0 = Monday, 1 = Tuesday, and so on.

Example Calculation

Using the formula above:


Since we use the convention that 0 = Monday, 1 = Tuesday, and so on, the result of 1.5 suggests that January 1st, AD 1, was a Tuesday.

Implications and Historical Context

Understanding that January 1st, AD 1, was a Tuesday provides interesting insights into the historical context and the continuity of the seven-day week cycle. It highlights the consistency of calendrical systems and their importance in historical chronology.

Impact on Historical Events

The determination of weekdays for historical dates can shed light on various historical events and cultural practices. For instance, knowing the day of the week can help historians understand the timing of religious observances, market days, and other significant events in ancient societies.

Cultural and Religious Significance

The seven-day week has deep cultural and religious significance. In Christianity, the association of specific days with religious observances, such as the Sabbath on Sunday, played a crucial role in shaping weekly rhythms and societal structures. Understanding the day of the week for January 1st, AD 1, helps contextualize early Christian practices and their evolution over time.


Determining the day of the week for January 1st, AD 1, involves a combination of historical knowledge, calendrical calculations, and astronomical methods. The Julian calendar, introduced by Julius Caesar, provides the framework for these calculations. By applying the Julian Day Number system, we can ascertain that the first day of the year AD 1 was a Tuesday. This finding not only satisfies historical curiosity but also underscores the continuity and significance of the seven-day week cycle in shaping human history and culture. As we explore the complexities of early calendars and their impact on historical events, we gain a deeper appreciation for the meticulous efforts of scholars and astronomers in preserving our understanding of the past.

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