Q:

Why is "Nous sommes" used for days of the week?

I've just been curious about this. Why is it that saying a day of the week, something like "It's monday" uses 'nous'? as in:

Nous sommes lundi.

Am I able to say it another way? is using: 

Aujourd'hui c'est lundi. / C'est lundi.

an acceptable format for a day of the week?
Furthermore, if that's the case, can I use

On est lundi.

in less formal contexts?
I just want to know which of these is correct to use, and why it is that 'nous' is used pour les jours de la semaine. 
Merci!

A:

Nous sommes lundi is a slightly formal/written way to tell which day of the week it is. One can also say Nous sommes [le] lundi premier septembre.
This French idiom can only be used in the first person plural.
You can't say Je suis lundi or ils sont lundi because the current weekday is expected to be the same for everyone.
The same idiom exists in Spanish: Estamos a lunes and Italian: Siamo [a] lunedì.
In English, while "We are Monday" will sound very foreign, one can say "We are late", "We are on time" or "We are in September" which are close.
Aujourd'hui, c'est lundi or simply c'est lundi are fully acceptable equivalents.
On est lundi is a less formal and more common way to say nous sommes lundi.

