kalendaro

This word is the reason we have a page called Festoj kaj la kalendaro, and the article is worth reading beside this entry: it is the Esperanto year, from Zamenhofa Tago on 15 December to the summer congress season, and it explains what the movement does with a calendar when no state prints its anniversaries on one.

Now the seam, because it looks like a good one and is not. Esperanto has a genuine collective suffix -ar-: a vortaro is a dictionary from vorto, an arbaro a forest from arbo, a homaro humanity from homo. So kalendaro invites the reading "a collection of kalendoj", and it would even nearly work, because kalendoj really is an Esperanto word - the Roman calends. But the word did not arrive that way. Kalendar- came in whole, from Italian calendario, German Kalender, English calendar, Russian and Polish and Yiddish forms of the same thing, all of them from Latin calendarium. The resemblance to -ar- is a coincidence of Latin spelling, and a genuine collective of calends would mean something quite different from a calendar.

And the history underneath is a gift. A Latin calendarium was not a calendar at all: it was an account book. The kalendae were the first day of the Roman month, the day debts came due and interest was reckoned, so the calendarium was the ledger where the moneylender wrote down who owed what. The word for the thing on your kitchen wall began life as the thing a moneylender kept. Kalendae itself comes from calare, to call out, because a priest announced the new moon aloud, which is also where intercalary comes from.

In use. Kalendara is the adjective, so kalendara jaro is a calendar year as opposed to a financial one. The Gregorian and Julian calendars are the gregoria and julia kalendaroj, and the difference between them matters more than you would think in Esperanto: Zamenhofa Tago is 15 December by the Gregorian reckoning, and the festival article explains why that footnote exists.