Quick answer
Classify the holiday first — fixed, moving or observed-shift — because each type needs a different calculation and only the first can be hard-coded.
Three kinds of holiday date
Fixed-date holidays fall on the same day and month every year: Christmas Day, New Year's Day, and most national days. These are the only ones that can safely be stored as a month-day pair.
Moving holidays follow a rule rather than a date. Easter is set by an ecclesiastical lunisolar calculation and can fall anywhere from 22 March to 25 April. Lunar New Year, Diwali, Eid and Rosh Hashanah follow their own calendars and shift substantially against the Gregorian year. Rule-based Monday holidays — "the last Monday in May", "the fourth Thursday in November" — are also moving, though far easier to compute.
Use this guide with the right tool: Open the Days Until Christmas Calculator for the number of days remaining until the next 25 December. If the question shifts, compare it with the Days Until New Year Calculator or the Countdown to Any Date Calculator.
The observed-date shift
The third category catches people out most. Many jurisdictions move a public holiday to the nearest weekday when it falls on a Saturday or Sunday, so the holiday date and the day off are different dates. The UK calls these substitute days; other systems call them observed holidays.
This matters for any calculation involving business days. If your working-day count uses the nominal date, it will be wrong in years when the shift applies — and it will be wrong in the direction that makes a deadline look safer than it is. Use the observed dates published by the relevant authority, not the nominal ones.
Regional variation inside one country
Public holidays are frequently sub-national. Scotland and Northern Ireland differ from England and Wales; Australian states differ from each other; US states add their own to the federal list; and many countries have regional or religious observances that apply to some employees and not others.
For a distributed team this means there is no single holiday calendar. Either maintain one per location, or agree explicitly which calendar governs shared deadlines. Assuming a shared calendar is how a deadline gets missed by exactly one day in one office.
Building a countdown that survives the year
For a fixed-date holiday, build this year's occurrence and roll forward only if it has passed. That single rule handles the whole year without special cases, including the holiday itself.
For anything moving or observed-shifted, compute it from the rule rather than from memory. Easter follows a lunisolar calculation that can be computed exactly for any year; nth-weekday holidays are simple arithmetic; substitute days follow a stated per-country rule. The calculators on this site work this way, which is why they are correct in years nobody has tabulated yet. Lunar-calendar holidays are the exception — those genuinely need a published table, refreshed as dates are announced.
Worked example
Swap in your own figures and the method carries over unchanged. What does not carry over is the source of authority, which is specific to your situation.
Common mistakes to avoid
- Hard-coding a moving holiday. Easter and lunar-calendar holidays follow rules, not dates. Source them from a published calendar each year.
- Using nominal dates for business-day counts. Substitute and observed days are the ones people actually take off. Use those.
- Assuming one holiday calendar per country. Regional variation is common. Agree which calendar governs before setting a shared deadline.
Use the right calculator
Pick the one whose inputs match the information you actually have. Each page explains what its result does and does not cover.
Frequently asked questions
Why does Easter move so much?
It is defined relative to a lunisolar ecclesiastical calculation rather than a fixed Gregorian date, giving a range of more than a month.
Do substitute days always exist?
No. Practice varies by country and by holiday. Check the rule for the specific jurisdiction.
Can I count holidays as business days?
Only if the relevant rule says so. Most business-day definitions exclude them, but the applicable holiday list is the part that varies.