Sexennial refers to something that occurs once every six years. However, if you are interested in finding synonyms for this word, you might consider using words like periodic, cyclical, or recurring. Other options might include "every sixth year," "six-year intervals," or "occurring once in six years." Depending on the context, you could also use terms like "regular," "repetitive," or "interval-based" to describe something that happens on a regular six-year schedule. Whatever synonym you choose, it should convey the idea of something that happens with regularity, but in a specific six-year pattern.