Even having looked in the OED I am still slightly unclear as to which contexts require the adjective ceremonious and which ceremonial.
The OED treatment of ceremonious is as below with some of the more recent examples. You will notice that in sense 1 it is equated to ceremonial. However sense 5 suggests a slightly different meaning.
- Pertaining to, or consisting of, ceremonies or outward forms and rites; = ceremonial adj., formal.
1737 D. Waterland Rev. Doctr. Eucharist 443 Ceremonious Observances.
- Full of ceremony; accompanied with rites, religious or showy.
1883 Manch. Examiner 14 Dec. 5/2 A statue has been raised to him..and there was a ceremonious unveiling.
- According to prescribed or customary formalities or punctilios.
1863 M. Howitt tr. F. Bremer Greece & Greeks II. xi, His..somewhat ceremonious politeness.
†4. According to the Ceremonial Law. Obs.
1656 S. Winter Serm. 120 The holiness of children, which some say was ceremonious.
- Of persons: Addicted to ritual observances (obs.); given to ceremony; punctilious in observance of formalities, esp. those of intercourse between ranks or persons.
1829 K. H. Digby Broad Stone of Honour: Godefridus xviii. 223 The ceremonious and ungrateful courtiers of Vienna.