tago

The greeting is Bonan tagon, with -n on both words, because it is the leftover object of an unspoken mi deziras al vi: you are wishing someone a good day, not merely naming one. That accusative survives in every fixed greeting, so Bonan nokton and Bonan matenon work the same way. Duration takes -n too: mi laboris la tutan tagon, no preposition needed. The root also gives ĉiutage for daily and tagiĝi for dawn, literally to become day.