Twelve months of thirty days and a short thirteenth — the Ethiopian calendar's near-twin.
The Coptic calendar descends from the ancient Egyptian one: twelve months of thirty days and a short thirteenth of five or six. The Ethiopian calendar shares that structure almost exactly, differing chiefly in its era, so the two run in step while numbering their years differently.
Its use today is largely liturgical, fixing the feasts of the Coptic Orthodox Church, and Coptic Christmas falls on the Gregorian 7 January — the same day as Orthodox Christmas in the Julian reckoning and Ethiopian Christmas in the Ethiopic.
Three calendars, three different routes, one Gregorian date. It is a good illustration of why a date alone tells you nothing about the rule that produced it.
Structurally near-identical and in step; the era differs, so the year numbers do not match.
Both put Christmas on 7 January Gregorian. They get there by unrelated arithmetic, and agreement on one date is not a shared system.
See also ethiopian calendar · julian calendar · epoch
We hold this as not held as its own rule_type — Coptic feasts in the corpus are recorded via ethiopic_date or as fixed Gregorian dates. See what we hold.