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History of the Gregorian Calendar

The Gregorian calendar is the most widely used calendar today. A regular year has 365 days, but every four years, a leap year adds an extra day to February, making it 29 days instead of 28. The months April, June, September, and November have 30 days, while the rest have 31 days.

The Gregorian calendar is an improved version of the Julian calendar, which was itself a change from the ancient Roman calendar. The ancient Roman calendar was originally based on the moon’s cycles. Later, the Romans created a 10-month calendar with 304 days, leaving about 50 days uncounted in winter. This caused seasons to shift over time, leading to more accurate calendar reforms.

The Republican calendar, used later in Rome, followed Greek ideas of 29.5 days per lunar cycle and 12.5 months in a solar year. To adjust the difference, extra months were added occasionally. In 46 BC, Julius Caesar reformed the calendar by setting 365 days per year and adding a leap day every four years to better match the solar year.

Even with these changes, the Julian calendar was still slightly off, losing about 11 minutes per year. By 1582, this had added up to a 10-day difference. Pope Gregory XIII fixed this by skipping 10 days, making October 4, 1582, followed by October 15, 1582. He also changed the leap year rule: century years (like 1700, 1800, 1900) are leap years only if they are divisible by 400. This made the calendar more accurate, reducing errors from 1 day every 128 years to 1 day every 3,030 years.

The Gregorian calendar took centuries to be fully adopted, but today it is the most widely used calendar worldwide.

Holidays

A holiday is a special day when normal activities like work or school are reduced or stopped. The meaning of “holiday” differs by region. In the U.S., national and cultural days off are called holidays, while personal time off from work is called vacation. In the UK and former British colonies, the word holiday can also mean paid leave.

Holidays usually honor an event, person, or tradition. Some holidays, like Christmas and New Year’s Day, are celebrated worldwide, while others are specific to a country. Even the same holiday may be observed differently in different places. Some holidays lead to complete business closures, while others only result in partial days off. For example, Carnaval do Brasil in Brazil almost shuts down the country for a week, except for some businesses related to the event.

This calculator mainly focuses on U.S. holidays, but you can add holidays from any country manually. Some holidays can also be excluded.

In the U.S., federal holidays are officially recognized by the government. On these days, non-essential government offices close, and federal employees get a paid day off. However, private companies decide for themselves whether to give employees a day off or holiday pay for working.

Some holidays, like New Year’s Day, are fixed and fall on the same date every year. Others, like Martin Luther King Jr. Day, follow a floating schedule—for example, it is observed on the third Monday of January every year. Similarly, Thanksgiving is on the fourth Thursday of November, so its date changes yearly.

Below are the dates for U.S. federal holidays in 2025 and 2026:

2025 U.S. Federal Holidays

2026 U.S. Federal Holidays