El uso de estamos

Question

When you say "colloquially", is it a less formal/ slang-like way to use it?And which region is it used?

Answer

Hola Alyssa,
using this sentence:

Estamos a 27 de noviembre. 
Today is 27th November.

is less formal than saying:

Hoy es 27 de noviembre.
Today is 27th November.

in the sense that the second is the most "standard, formal and literal" way to say it, and the first one using "estamos a" would be more informal and idiomatic. 
But both can be said in any context and situation. You could use both when talking to a friend, and you could say both when you are in a more formal/serious situation, e.g. a meeting with clients.
I hope this helps,
Saludos,
Inma