About this data

The dates on Calendar USA should be dates you can rely on. This page explains where they come from, how we calculate them, how often we check them and what to do if you spot a mistake.

Where does the data come from?

Federal holidays

We calculate the public holidays ourselves using the statutory rules for the United States. We check those rules against the official source below: the government page or the legislation that sets out the holidays.

School holidays

School holiday dates come from official publications by the government, the education ministry or the region or local authority that sets the holidays. Every date in our database links to the source it came from. The sources we use for the United States are listed below.

64 sources · Last checked: October 12, 2026.

Week numbers

Week numbers follow the international standard ISO 8601. No data source is needed: every week number follows directly from the calendar.

Moon phases

We calculate the moon phases ourselves with astronomical formulas. No external data source is used.

How do we calculate the dates?

Federal holidays

Each holiday is stored in our code as a rule: a fixed date, a number of days after Easter or, for example, the first Monday of a month. Easter itself is calculated with the algorithm for the Gregorian calendar. Every rule has automated tests with the dates of at least three known years and a reference to its source. That lets us show the holidays for the years ahead without typing in individual dates.

School holidays

School holidays cannot be calculated: they are set for each school year. We take the dates from the source and show them by region. Dates that have not yet been officially confirmed are marked as provisional. Below every table you can see which source we used and when the dates were last checked.

Week numbers

We show two week numbers. Under ISO 8601 a week starts on Monday, and week 1 is the week containing the first Thursday of the year, so a year has 52 or 53 weeks. In the common US system a week starts on Sunday and week 1 is the week containing January 1, as in Excel's WEEKNUM function and Outlook's default. Both calculations are covered by automated tests, including around the turn of the year.

Moon phases

New moon, first quarter, full moon and last quarter are calculated with the method from Jean Meeus, Astronomical Algorithms (chapter 49). Times are shown in Eastern Time and are accurate to within a few minutes.

How often do we check the data?

Federal holidays

Holiday rules rarely change, and only by a decision of the government. When they do, we update the rule. Every change is tested before it goes live.

School holidays

Every hour an automated process checks part of our sources for changes, so each source is checked regularly: weekly, monthly or quarterly, depending on how often it changes. When the content of a source changes, an editor reviews the new information and updates the dates.

Week numbers

The ISO standard does not change, so the week numbers themselves never need rechecking. The pages are refreshed daily, so today's week number is always right.

Moon phases

The calculation is covered by automated tests against the moon phases published by the U.S. Naval Observatory.

Spotted a mistake?

Is a date wrong, or is something missing? Let us know via the contact form, ideally with a link to the source. We review every report and correct the data where needed.

Report a mistake via the contact form