Anniversary Almanac

Why a fixed date gets stuck

A common year is 365 days, and 364 is exactly 13 × 28. So thirteen full cycles land one day short of the year — a 28-day cycle slips against the calendar by a single day each year, two after a leap day. That is slow enough that an anniversary stays in roughly the same part of your cycle for years at a time, then spends the following years in the opposite part.

It is also unusually specific to 28. A 27-day cycle moves 14 days a year against the calendar and a 29-day cycle moves 17 — both scatter the date within a single year. Only cycles of 26 and 28 days divide 364 evenly, which is why this pattern shows up for some people and never for others.

Why the phases shift when you change the length

The luteal phase is biologically about 14 days whatever your total cycle length, so nearly all the variation lives in the first half. This almanac places ovulation roughly 14 days before your next expected period, which is why raising your cycle length stretches the follicular stretch and leaves the later phases where they are. It is the same model behind our cycle calculator.

What “capacity” means here

The shaded rows are the low-capacity window: the late luteal stretch running straight into menstruation. They sit either side of the cycle boundary, so they are one unbroken run rather than two separate ones. It describes how much a day has spare to give — not mood, and never a prediction of anyone's behaviour. A low reading is an argument for a quieter plan, never for less care.

What to do with it

The date on the certificate never changes. The day you mark it can — and a shift of a week or two is often enough to move a decade of anniversaries out of the hardest window. Drag the slider to see the difference, or let the tool find the shift that gives you the most across all 22 years.

Elsewhere on the site: work out when your next period is due, find how long your cycle actually is, read what your hormones do in each phase, or browse the full Cycle Library.