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Calendar systems and the dates that move
Calendar systems and the dates that move: why Orthodox Christmas falls on 7 January, when Easter lands in any year and why the two churches disagree by up…
This launch guide maps the topic, the source base and the practical questions. Check the linked official authority before acting on a rule, deadline, price or safety requirement.
What this guide covers
Why Orthodox Christmas falls on 7 January, when Easter lands in any year and why the two churches disagree by up to five weeks, the Hijri and Hebrew year converters with their local sighting conventions, leap years and leap seconds, the days that vanished in the Gregorian reform, and why the week starts on Monday here and Sunday there. One page per system plus a computed per-year table from 1900 to 2100. A focused example is "when is Orthodox Easter, year by year".
How to use it
Use Calendar systems and the dates that move to compare stable place facts and official facilities. Live opening, access and disruption information should always be confirmed with the operator before travel.
The page keeps the main question, its evidence base and its limits together in one canonical place. That makes it useful as a quick orientation and avoids splitting a broad subject into shallow pages before each narrower answer has enough verified material of its own.
Search vocabulary in four languages
Terminology changes across borders. These are example phrases people use for the same subject in Estonian, Finnish, Russian and English; they are included as a practical vocabulary aid, not as hidden keywords.
| Language | Example phrase |
|---|---|
| Estonian | miks õigeusklikel on jõulud 7. jaanuaril |
| Finnish | milloin pääsiäinen 2028 |
| Russian | почему православное Рождество 7 января |
| English | when is Orthodox Easter 2028 |
Evidence and provenance
The evidence map below records the primary, official or openly licensed material chosen for this guide. It is deliberately visible so a reader can tell the difference between a rule published by an authority, a measured dataset and an editorial explanation.
Source: none, computed (Gauss and Meeus Easter algorithms, tabular Hijri, arithmetic Hebrew calendar); reform dates cited from published primary sources. · checked 2026-09-09
Source names are preserved even where an authority changes its web address. No figure is added to this launch page unless it is already present in the cited source description or can be computed from a named dataset.
Scope and boundaries
/fasts computes the fasting days that follow from these dates; this is the machinery that produces them.
Connected TempoLife guides
The following paths are the intentional hand-offs named in the brief. They keep adjacent subjects separate while making the next useful reference easy to find.
Why it belongs on TempoLife
/fasts.
Frequently asked questions
What does this guide cover?
Why Orthodox Christmas falls on 7 January, when Easter lands in any year and why the two churches disagree by up to five weeks, the Hijri and Hebrew year converters with their local sighting conventions, leap years and leap seconds, the days that vanished in the Gregorian reform, and why the week starts on Monday here and Sunday there. One page per system plus a computed per-year table from 1900 to 2100. A focused example is "when is Orthodox Easter, year by year".
What evidence does it use?
none, computed (Gauss and Meeus Easter algorithms, tabular Hijri, arithmetic Hebrew calendar); reform dates cited from published primary sources.
Where does this guide stop?
/fasts computes the fasting days that follow from these dates; this is the machinery that produces them.
Turn the reference into a routine
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