A note on American English:
- Both centennial and centenary can be either an adjective (“every century”) or a noun (“centennial celebration”).
- In writing, it is commendable to reserve centennial for the adj. and centenary for the n., but it is a distinction that only connoisseurs will care about. (CMOS 17 is silent; Webster’s cross-references each to the other.)
- In speech, my impression is that Yanks frequently say “centennial” for both the adj. and n.
- British centennium, which means “centenary” (n.)—or, as I learned once to my amazement, sometimes even “century” in Oxbridgese—is not a word in American.
- It is interesting to compare the subtle difference in the sense of centenary as a centennially recurring celebration of a significant historical date, and the once-a-century, but also once only celebration of a given century’s passing: the latter is a special case of the former. (Note, in this connection, that one never celebrates a century’s bicentenary!) The OED distinguishes both senses, but lists them both under its definition 2b. Webster’s sensibly avoids such flyspecks.