Why plural days and nights in Spanish greetings?

R.R. points out that many European languages have a greeting that means "good day" — German "guten tag", Dutch "goeden dag", Swedish "god dag", French "bonjour", Italian "buon giorno", Portuguese "bom dia", Catalan "bon dia", etc. — and asks why (only?) in Spanish, the corresponding phrase is plural: "buenos dias". And also "buenas noches", "buenas [...]