Sources behind these design choices
The US Office of Personnel Management explains observed federal holidays. The UK National Archives provides guidance for reading historical documents. These resources support the date distinctions described above; they are not endorsements of this website.
How the site is built
The calendar engine. Every calendar, holiday date, moon phase and season on this site is calculated by code, not typed by hand. Federal holidays follow 5 U.S.C. 6103 and the Office of Personnel Management's weekend rules. Easter uses the standard Gregorian method, Jewish, Islamic and Lunar New Year dates come from the Hebrew, Umm al-Qura and Chinese calendars, and moon phases and seasons use the published algorithms of astronomer Jean Meeus. An automated test suite checks these results against known, published dates every time the site is built.
The history. Year timelines, event articles, holiday histories and guides are researched from government archives, official organizations, universities and established news services. Each article lists its sources at the end, and wording is original to Old Calendars. Our editorial policy explains how pages are researched and corrected.
Your privacy. The calendars and calculators work without an account. Dates you type into a calculator stay in your browser.