Ceremonialistic refers to something that is done according to a strict set of rules and traditions. Its antonyms, on the other hand, are words that describe actions or behaviors that are informal, spontaneous, or casual. Some of the antonyms for ceremonialistic include casual, informal, unceremonious, spontaneous, and natural. These words describe actions and behaviors that are unconstrained by strict traditions or formalities. For example, an informal gathering with friends is an antonym for a formal and ceremonial occasion. Similarly, a spontaneous dance party is an antonym for a structured and choreographed dance performance. In essence, anything that is unrestrained by formality can be considered an antonym for ceremonialistic.