The Spanish word 'efeméride' comes from the Latin 'ephemeris', which was borrowed from Ancient Greek 'ephemeris' (ἐφημερίς) meaning 'diary' or 'daily record'. The Greek word was formed by combining two elements: 'epi-' (ἐπί) meaning 'upon' or 'concerning', and 'hemera' (ἡμέρα) meaning 'day'. So literally, it meant something like 'upon/concerning the day'. While it originally referred to a diary or daily record, in Spanish it has evolved to specifically mean a notable event that occurred on a particular date - the kind of historical event you might find in a 'This Day in History' calendar.

The word 'efímero' (ephemeral, short-lived) is a related Spanish word that comes from the same Greek roots. Both words share the connection to 'hemera' (day), with 'efímero' literally meaning 'lasting for a day' and thus coming to refer to anything temporary or short-lived.

English speakers might recognize this word's connection to 'ephemeral' (lasting for a very short time) and 'ephemeris' (a table giving the positions of celestial bodies on each date). These words share the same Greek ancestry as 'efeméride'. While 'ephemeral' emphasizes the brief, temporary nature of something lasting just a day, and 'ephemeris' maintained the sense of a daily record (specifically of astronomical positions), the Spanish 'efeméride' focused on the aspect of notable daily events worth recording.