What is a 28 day calendar called

A 28-day calendar is sometimes called a "lunar calendar" when it tracks moon cycles, or more specifically, it may be referred to as a "menstrual cycle calendar" when used to track fertility and reproductive health, since the average menstrual cycle approximates 28 days. In business and scheduling contexts, some organizations use 13 periods of exactly 28 days each (totaling 364 days) as an alternative calendar system, though this lacks a widely recognized formal name and is not the international standard.

The most well-known formal 28-day calendar system is the "International Fixed Calendar" or "Cotsworth Calendar," proposed in the early 20th century but never widely adopted. This system divides the year into 13 months of exactly 28 days each (four complete weeks per month), plus one or two additional days outside the regular month structure. While mathematically elegant and highly regular, this system failed to gain traction against the entrenched Gregorian calendar. Today, 28-day tracking systems appear primarily in specialized contexts—fertility apps, shift work schedules, and certain business accounting methods—rather than as a replacement for the standard 12-month calendar. Understanding these alternative systems reveals ongoing human efforts to create more regular, predictable timekeeping structures, even though practical implementation remains limited by the momentum of established calendar conventions.