Ceremonial and ceremonious both suggest formality, but they are not interchangeable.
Ceremonial usually describes something connected to a ceremony, ritual, or official public occasion. It often applies to objects, roles, clothing, language, or actions that are part of a formal event. For example, The mayor wore a ceremonial robe for the opening.
Ceremonious describes a person’s manner, behavior, or tone when it is very formal, sometimes so formal that it feels stiff or excessive. For example, She became ceremonious when greeting the visiting judge.
- Use ceremonial for things linked to rituals, traditions, or official proceedings.
- Use ceremonious for behavior that seems carefully formal or overly polite.
A quick contrast can help: a ceremonial dinner is an official or ritual event, but a ceremonious host behaves with noticeable formality. If you mean the event itself or something used in it, choose ceremonial. If you mean the manner of a person, choose ceremonious.