The Spanish word 'calendario' (calendar) comes from the Latin word 'calendarium', which originally meant 'account book'. This Latin term derived from 'kalendae', meaning the first day of the month in the Roman calendar. Going even further back, 'kalendae' comes from the Latin verb 'calare', which meant 'to call out or proclaim' - this is because in ancient Rome, priests would publicly announce or 'call out' the first day of each new month.

A common related Spanish word is 'calenda', which refers specifically to the first day of each month in the ancient Roman calendar, maintaining a direct connection to the Latin 'kalendae'.

Several common English words share this etymology with 'calendario'. The most obvious is 'calendar' itself, which followed a similar development path through Latin. We also see this root in 'calends' (the first day of the month in the Roman calendar) and in words like 'intercalate' (to insert an extra day or month into the calendar). The word 'calculate' also shares this origin, as Roman accounts were typically settled on the calends of each month.