New Year happens all year: how calendars disagree

A calendar has to reconcile three natural cycles that do not divide evenly into each other: the day, the lunar month of roughly 29.5 days, and the solar year of roughly 365.24 days.

Twelve lunar months come to about 354 days, eleven short of a solar year. Every calendar system is a different answer to that mismatch, and the answer determines when the year turns.

Solar calendars

Track the sun and ignore the moon. Months become administrative divisions with no astronomical meaning.

The Gregorian calendar is the widely used civil example. It handles the fractional day with a leap year rule refined from an earlier Roman system, and the refinement — skipping leap years in most century years — was the reform that produced the switch from the Julian calendar.

That switch happened at different times in different countries, over a span of centuries, with the result that dates in historical records from the transition period require care about which system is being used.

Other solar calendars exist, including several used for agricultural and religious purposes, and some are more astronomically accurate than the Gregorian.

Lunar calendars

Track the moon and let the year drift against the seasons.

The Islamic calendar is the main example in wide use. Twelve lunar months, about 354 days, no intercalation. The consequence is that the calendar moves backward through the solar year by roughly eleven days annually, so that any given month cycles through all the seasons over about thirty-three years.

This means Ramadan falls in high summer in some years and in winter in others, with substantial practical differences in the length of the daily fast depending on latitude.

Traditionally, the start of each month is determined by the sighting of the new crescent moon, which introduces genuine local variation — different regions may begin a month on different days depending on visibility, and there is longstanding discussion about calculation versus observation.

Lunisolar calendars

The compromise: follow the moon for months, and periodically insert an extra month to keep the year aligned with the seasons.

The Hebrew calendar does this on a fixed nineteen-year cycle containing seven leap years, a cycle that works because nineteen solar years and 235 lunar months come to almost exactly the same length.

The Chinese calendar operates similarly, with intercalary months determined by the sun's position, which is why Chinese New Year falls between late January and late February rather than on a fixed date.

Hindu calendars, of which there are several regional systems, are lunisolar with regional variation in the rules, which is why festival dates differ between parts of India.

Why the year starts where it does

Different systems anchor to different events.

Astronomical points — an equinox or a solstice. Several traditions begin the year at or near the spring equinox, which has an intuitive logic: the year begins when things start growing.

Agricultural moments — harvest, the start of a rainy season, the point when a river floods. Calendars anchored to these are anchored to what actually mattered locally.

Religious events, commemorating something specific.

Administrative convenience. January as the start of the year has a Roman administrative origin rather than an astronomical one, and it took a long time to become standard even in Europe — several countries began the legal year in March well into the early modern period.

The practical consequences

Multiple calendars in use simultaneously is the normal condition in much of the world, not an exception. A civil calendar for administration, a religious calendar for observance, and sometimes a traditional agricultural calendar for planting.

People manage this without difficulty, which is worth noting given how much confusion it seems to cause in software design. Date handling that assumes one calendar, or that assumes a year has twelve months of predictable length, fails in ways that are annoying for a great many users.

What all of them share

Whenever the year turns, the ritual content is remarkably consistent.

Cleaning — houses swept, debts settled, disputes resolved before the new year. Specific foods, frequently symbolic of prosperity or continuity. Noise, to drive something away. Visits to relatives and to graves. Divination about the year ahead. A moment of transition marked precisely.

Those elements recur across calendars that have nothing in common astronomically, which suggests that what people want from a new year is not determined by which day it falls on.