Calendar knowledge
Guides that explain the date, not just display it
Plain-English answers, worked examples and reference tables for the questions behind every calendar: why years repeat, how holidays are set, what the moon is doing and how to read old dates.
7 guides
Calendar basics
What Day of the Week Was That Date?
Use the instant day-of-week calculator, then learn a reliable mental method for checking birthdays, anniversaries and historical dates by hand.
Read the guideWhy Calendars Repeat and Which Years Match
A year calendar repeats when another year starts on the same weekday and has the same leap-year status. Only fourteen basic Gregorian year layouts are possible.
Read the guideLeap Year Rules, Century Exceptions and February 29
A leap year adds February 29 to keep the civil calendar aligned with the seasons, but century years must pass an extra divisible-by-400 test.
Read the guideISO Week Numbers: Week 1, 53-Week Years and US Weeks
ISO week numbers count Monday-to-Sunday weeks, and week 1 is the week containing the year's first Thursday. Many US calendars and spreadsheets number Sunday-based weeks instead, so the two systems can disagree.
Read the guideChinese Zodiac Years: Find Your Animal and Element by Birth Year
Each Chinese zodiac year pairs one of 12 animals with one of five elements, and it begins at Lunar New Year rather than January 1. This guide explains the cycle, the date rule and how to work out your sign step by step.
Read the guideFriday the 13th: Why Every Year Has One to Three
A month has a Friday the 13th whenever it starts on a Sunday. That one rule explains why 2026 has three, why no year can have none, and why the 13th lands on a Friday slightly more often than on any other weekday.
Read the guideDay of the Year and Julian Dates: How to Read YYDDD Codes
A day-of-year number counts from 1 on January 1 to 365 or 366 on December 31. Combined with the year it makes the YYDDD codes often called Julian dates, which are not the same thing as the astronomical Julian Day Number.
Read the guide2 guides
Holidays
How Easter's Date Is Calculated: The Computus Explained
Western churches date Easter from a fixed March 21 equinox and a tabulated ecclesiastical full moon. This guide explains the computus, the March 22 to April 25 range, Orthodox Easter and a step-by-step calculation.
Read the guideUS Federal Holidays: Observed Dates and Closures
Federal law names 11 annual public holidays for federal employees, but the calendar date, observed day and closure policy are not always the same.
Read the guide2 guides
Astronomy
Moon Phases Explained: The Lunar Cycle, Full Moon Names and Blue Moons
The Moon cycles from new to full and back in about 29.53 days. This guide explains the eight phases, the arithmetic behind the cycle, how phase times are computed, and the folk names, blue moons and supermoons you see on calendars.
Read the guideEquinoxes and Solstices: Dates, Times and Why They Shift
Equinoxes and solstices are precise moments in Earth's orbit, not whole days. This guide explains what they are, why their dates move around from year to year, why the seasons are unequal in length, and how daylight changes across the United States.
Read the guide1 guide
Time & clocks
4 guides
History & research
Gregorian Calendar History: From Caesar to the 1752 Switch
The Gregorian calendar replaced Julius Caesar's Julian calendar in Catholic Europe in 1582. Britain and its colonies switched in September 1752, which is why colonial dates such as George Washington's birthday have Old Style and New Style versions.
Read the guideGenerations by Birth Year: Boomers, Gen X, Millennials and Gen Z
Generation labels are research shorthand, not official categories. This guide gives the birth-year ranges most US researchers use, explains where the names came from, and shows why someone born near a boundary can land in two different generations.
Read the guideWhat Happened on the Day You Were Born: A Research Guide
A step-by-step plan for researching any birthday or past date, from the weekday and nearby holidays to newspaper archives, Billboard charts, NOAA weather records and vital records, plus how to build a keepsake page.
Read the guideWhere Month and Weekday Names Come From
The month names come from Rome: gods for the first few, two rulers for July and August, and plain numbers for September through December. The weekday names come from the seven classical planets, with four Roman gods swapped for Germanic ones in English.
Read the guide2 guides
Printing & planning
How to Print a Year Calendar or Save It as a PDF
Use the Print button on a year page for a clean calendar-only layout, then choose paper size, orientation, scaling or Save as PDF in your browser.
Read the guideHow to Count Business Days Between Two Dates
A US business day is normally a weekday that is not a federal holiday. This guide shows how to count them by hand and in a spreadsheet, how many fall in a typical year, and where banks, courts and payroll calendars use different rules.
Read the guideHow these guides are made
Each guide answers one practical question, shows the rule or calculation behind it and links the answer back to real year calendars. Historical and legal claims cite primary or authoritative sources, listed at the end of every guide. Worked date examples are checked against the same tested date engine that draws every calendar on this site.