Ceremonial, ceremonious, formal, conventional, solemn are comparable when they mean characterized or marked by attention to the forms, procedures, and details prescribed as right, proper, or requisite.
Both ceremonial and ceremonious imply strict attention to and regard for what is prescribed by the etiquette or tradition of a court or of polite society, by the ritual of a church, or by the formalities of the law for a ceremony or a proceeding. They are sometimes interchangeable, but more often ceremonial distinctively suggests the existence of and conformance to an elaborate, prescribed, and usually ritualistic code of procedures, while ceremonious stresses elaborate, often punctilious and dignified procedures or, in reference to persons, a tendency to formality and ceremony; thus, a wedding is a ceremonial occasion but not one at which the participants need be excessively ceremonious.
Formal in comparison to ceremonial suggests set form or procedure rather than external ceremonies and in comparison to ceremonious, formal suggests stiffness, restraint, decorousness rather than impressive dignity or punctiliousness.
Conventional applies to whatever is in accord with or governed by recognized, frequently artificial conventions or standards; it connotes lack of originality or independence.
Solemn is used in relation to religious observances or services and to certain acts the conduct of which is prescribed by law. The term implies, usually, strict attention to every detail that is prescribed or allowed by the ritual of the church or by the formalities of the law; thus, a solemn Mass is one in which the full liturgy is followed; a solemn feast is one celebrated not only by the full liturgy but by such other ceremonial observances as processions and pageants.