The Gregorian calendar is the most widely used nowadays. Within this calendar, a conventional year is made up of 365 days, with a leap day added to February during leap years. The months of April, June, September, and November have 30 days each, while the remainder have 31 days, with the exception of February, which has 28 days in a standard year and 29 in leap years.
The Gregorian calendar is a revised version of the Julian calendar, which was derived from the ancient Roman calendar. The ancient Roman calendar was thought to be an observational lunar calendar based on the moon's phases. The Romans were supposed to have used a 10-month calendar with 304 days, leaving an additional 50 or so days for a disorganized winter. This calendar caused the summer and winter months to become utterly misaligned, prompting the use of more accurate calendars.
The Republican calendar, later used by Rome, was based on Greek calendars in that it assumed 29.5 days in a lunar cycle and 12.5 synodic months in a solar year, which aligned every fourth year with the inclusion of the intercalary months in January and February. From this point forward, numerous attempts were made to align the Republican calendar with the solar year, including adding an extra month to specific years to compensate for the shortage of days in a given year. Julius Caesar further modified the calendar in 46 BC, introducing an algorithm that eliminated calendars' reliance on new moon observations. In order to accomplish this, Caesar added ten days to the Republican calendar, bringing the total number of days in the year to 365. He also inserted a leap day every fourth year in an attempt to further align the Roman calendar with the solar year.
Despite all attempts, the Julian calendar required additional modification because it deviated by around 11 minutes per year from the equinoxes and solstices. By 1582, this had resulted in a 10-day divergence from what was planned. Pope Gregory XIII handled the issue by simply skipping ten days, making the day following October 4, 1582, October 15. An alteration was also made to the Julian calendar algorithm, changing which century years were considered leap years. Under the Gregorian calendar, century years that are not divisible by 400 are not considered leap years. These adjustments lowered the inaccuracy from 1 day in 128 years to 1 day in 3,030 years, based on the current mean solar year value.
The Gregorian calendar was gradually adopted over centuries, and despite numerous suggestions to modify it, it remains the most widely used dating system in the world.
A holiday is a day set aside, either by custom or by legislation, to halt or reduce usual activities such as going to work or school. The term "holiday" is understood differently depending on the locale. Paid leave in the United States is commonly referred to as "vacation," whereas national, religious, or cultural holidays are primarily referred to as "holiday." In some areas, such as the United Kingdom or former British colonies, the term holiday can also refer to paid leave.
Holidays are generally intended to memorialize a culturally or religiously significant event, person, or group. Although certain holidays, such as Christmas and New Year's Day, are widely celebrated around the world, most countries have their own set of holidays that are unique to their country. Even the same holidays may be observed differently within countries: some may see a complete suspension of normal daily activities, while others may only get partial days off. Certain countries observe holidays, which effectively shut down practically all companies. For example, in Brazil, Carnaval do Brasil lasts nearly a whole week, during which only industrial output, retail outlets, and carnival-related enterprises operate.
This calculator is primarily designed for US holidays; however, holidays specific to a particular country can be added manually. Certain holidays may also be excluded. To be more explicit, federal holidays in the United States are government-recognized holidays on which non-essential federal government agencies are closed, and all federal employees are compensated. This is only sometimes true in the private sector, and which federal holidays a private sector employee receives is primarily at the discretion of the employer. In rare situations, employees who are compelled to work on a federal holiday may be compensated with holiday pay in addition to their usual income.
Certain holidays, such as New Year's Day, are referred to as "fixed holidays" because they occur on the same date each year. Others, such as Martin Luther King Jr.'s birthday, may not have a fixed date since they appear on a "floating Monday"; in this case, the holiday falls on the third Monday of January. Another widely observed holiday in the United States, Thanksgiving, falls on a "floating Thursday," the fourth Thursday in November; therefore, the dates of these festivals shift from year to year. The two tables below show the dates of federal holidays in the United States for 2024 and 2025.