Calendar insights
The leap-year boundary
Why February divides the calendar into two useful segments.
One extra day changes the distance
In an ordinary year, the number of days between a January date and a March date differs from that distance in a leap year. The extra day is February 29. A one-day change is enough to break a weekday relationship because weekdays repeat in cycles of seven, not eight.
Keep comparisons on one side
Always Happens uses one set of groups for January 1 through February 28, and a separate set beginning March 1. Within a segment, the distance between two month-and-day dates stays constant. February 29 is not assigned to an ordinary permanent group. This prevents a short-lived coincidence from being described as something that always happens.
What a cross-boundary match means
Two dates on opposite sides of February may still fall on the same weekday in a particular year. That is a valid year-specific observation. It is simply a different statement from a permanent month-and-day relationship. A clear result should tell you which kind of observation you are seeing.
When sharing the result
Keep the condition in the explanation. “These dates share a weekday this year” and “these dates share a weekday every year” are different promises. The more precise wording makes the discovery more useful and gives your friend a way to check it independently.
Always Happens editorial draft · calendar claims are covered by automated tests; human editorial review is required before public launch.