1) Complemental 2) Formal 1) Conventional 2) Formal 3) French word used in English 4) Like a solemn observance 
• (a.) According to prescribed or customary rules and forms; devoted to forms and ceremonies; formally respectful; punctilious. • (a.) Consisting of outward forms and rites; ceremonial. [In this sense ceremonial is now preferred.]
Cer`e·mo'ni·ous adjective [ Confer French cérémonieux , Latin Caerimoniosus .] 1. Consisting of outward forms and rites; ceremonial. [ In this sense ceremonial is now preferred.] « The ceremonious part of His worship. South. » 2. A...
[adj] - rigidly formal or bound by convention
conventional adjective rigidly formal or bound by convention; `their ceremonious greetings did not seem heartfelt`
carefully observant of ceremony; formally or elaborately polite: He greeted his rival with a ceremonious display of friendship. · pertaining to, marked by, or consisting of ceremony; formal: a ceremonious reception.