Inserting a day or a month to keep a calendar in step with what it tracks.
Intercalation is the insertion of an extra day, month or second to reconcile a calendar with the cycle it is meant to follow. Every calendar needs it, because no astronomical period is a whole number of any other.
The Gregorian leap day is intercalation. So is the thirteenth month a lunisolar calendar inserts seven times in nineteen years, and so is the leap second occasionally added to civil time.
The rules are where calendars differ most, and where date arithmetic goes wrong most quietly. A system that assumes twelve months, or 365 days, or 86,400 seconds, is assuming its own calendar's intercalation rule is universal.
A leap year is one instance of intercalation — the Gregorian one. Intercalation is the general mechanism, and a lunisolar leap month is a different instance of it.
See also leap year · leap second · lunisolar calendar · gregorian calendar
We hold this as handled inside each rule_type rather than stored as a field. See what we hold.