Query: Why the plural in Spanish "Buenas dias," etc.? (OT)

[Gerald Cohen]

    This is off topic, but perhaps someone on the list knows the
answer: Why does Spanish use the plural for Buenas dias (Good day),
Buenas tardes (Good afternoon) and Buenas noches (Good night)?  I
know of no other language that does this.

    Any help would be much appreciated.

Gerald Cohen