【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.