A form performed to mark that something real has changed. Ceremony does not cause the change; it witnesses it.
Latin caerimonia (sacred rite, reverent observance), of disputed origin — sometimes proposed as Etruscan, sometimes connected to carus (dear); the deeper root is not settled, but Rome's use of the word for solemn, formal ritual is.
The change already happened before anyone gathered for it — a wedding does not marry two people any more than a graduation teaches a degree, so a ceremony held here is evidence offered to a room, never a mechanism the room depends on.