https://timesofindia.indiatimes.com/life-style/books/features/word-of-the-day-septennial/articleshow/129837615.cms
Word of the Day: Septennial | - The Times of India
Some instances do not occur annually but have a longer and more spaced-out interval. Consider traditions, celebrations, or even reviews that occur onc.
word of the daytimesindia
https://digital.library.yale.edu/catalog/10727796
The Brentford race for the Middlesex septennial plate [graphic] - Yale University Library
Dent, William, active 1783-1793, printmaker; April 22d, 1784.
yale universitybrentfordracemiddlesex