The kwiziq page "Ser vs Estar in Spanish: Using ser in Spanish (not estar) to talk about time, days, dates and seasons" (Ser vs Estar in Spanish: Using ser in Spanish (not estar) to talk about time, days, dates and seasons) says
Hoy es lunes. = Today is Monday. ("Today" is singular.)
but
Son las tres de la tarde. = It's three o'clock in the afternoon. ("It" is plural.)
Why are these different?
Hola Phil
The first sentence "Hoy es lunes" is an impersonal sentence with no subject and the verb is used in singular- "hoy" is not the subject, hoy is an adverb of time (hoy, ayer, mañana...)
There are other impersonal sentences where you use the verb in singular, e.g: Llueve mucho (It rains a lot), Hace frío (It's cold)...
In the second sentence: "Son las tres" we are using the verb ser in the plural form because that's how we use it when we say "what the time is", we use it in plural from 2 pm onwards: Son las dos, son las dos y media, son las tres, son las cinco y cuarto... (always "son") - however we use it in singular if the time is 1 o'clock or up to 2 pm: Es la una, es la una y cuarto, es la una y media, es la una menos diez...
It's a bit confusing because in English you always say "It's..." whatever the time: it's 1 o'clock, it's ten past five...
I hope it clarified it.
Saludos
Inma
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