I am not convinced that bidecennial would be understood by any other than a handful of BE speakers. Even then there would be much head-scratching and wondering if it meant that something happened twice every ten years or once every other ten years. Every twenty years would have to be vigintennial. I think every twenty-five years would have to be vigintiquinquennial.There is one word for a 20-year anniversary or something that happens once in 20 years, bidecennial. Is there such a word for 25 years too, please?
Thanks a lot!