n the scientific name of the marigold you recognize the Latin word "Calenda", you recognize that our 'Calendar' in, and indeed 'kalends' the name given to the first day of the month (kalends maia' is not only the title of the oldest secular Western European song whose music has been preserved, but it is also "the first of May). By this I immediately landed on one of my pet: I usually emphasize that a plant a 'scientific' or 'botanical' has name, not a 'Latin', as many names have their roots in the Greek language. But Calendula is one of those exceptions that prove the rule, and is derived from Latin.
But what is now "the first day of (each) month 'to do with Marigolds? Perhaps you've already noticed Marigolds, although they are one-year, very able to flourish long in any case in mild winters and milder climates ... and Marigold so flourished in the Mediterranean climate in just about every month, and possibly on each first day ... "officinalis" is a term that you find at a lot of (prescription) herbal generic, and it actually refers to the healing power of the plant: The 'pharmacy' is indeed the work of a pharmacist, a pharmacy lab so, or more commonly just a pharmacy. (In Belgian legislation is called a pharmacy, incidentally still a pharmacy.)