The word calendar was first used in 1275, when it was spelled as kalender. Other alternations included kaluneder, kalendere, kalandar, and calandar until calendar was standardized during the eighteenth century. Through Old French, that traces to the Latin word kalendarium (sometimes calendarium), which meant "account book" and was associated with months because it was named after the kalandae (or calendae), the first day of the Roman month. This is because all debts were due during that time. Kalandae derives from the reconstructed Proto-Indo-European word kele, which meant "shout" because, at the start of the calends, priests were supposed to go around shouting that it was a new month and how long it was to the nones (the next part of the month). Usage of the word calendar over time has been steadily increasing over time, reaching its highest point so far in the year 2001.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.