355day

The term "355day" refers to a calendar system where a standard year is composed of 355 days. This is a departure from the common Gregorian calendar which has 365 or 366 days. Calendars with 355 days are typically lunisolar, meaning they attempt to track both the moon's phases and the sun's cycle. The length of a lunar month is approximately 29.5 days. Twelve lunar months would therefore total around 354 days. To reconcile this with the solar year, which is approximately 365.25 days, adjustments are necessary.

A 355-day calendar often inserts an intercalary month periodically to keep the lunar cycles synchronized with the seasons. This intercalary month, sometimes called a "leap month," is added at specific intervals to realign the lunar calendar with the solar year. Without such adjustments, the months of a purely lunar calendar would drift significantly through the seasons over time. Historically, several cultures have utilized or are still utilizing calendars based on a 355-day year, often with modifications. The traditional Islamic calendar is a purely lunar calendar with 12 months, totaling approximately 354 or 355 days depending on whether the year is a leap year within its system, and it does not attempt to synchronize with the solar year, causing its festivals to cycle through all seasons. However, many ancient lunisolar calendars, such as those used in Mesopotamia or early Greece, employed variations of a 355-day base with added intercalary months to maintain seasonal alignment.