There are several synonyms for the phrase "once every twelve months." Some of the most common ones include "annually," "per annum," "yearly," "once a year," and "once annually." These phrases all refer to something that occurs once every twelve months, whether it is an event, a payment, or a specific milestone. Using these synonyms can help to vary your language when talking about events that occur on an annual basis and can help to keep your writing or speech more interesting and engaging. No matter which synonym you choose, the meaning remains the same: something that happens once a year.