Here are a few ways to respond:
Formal:
* Hoy es [day of the week]. (e.g., Hoy es lunes.)
* Estamos a [date]. (e.g., Estamos a 15 de febrero.)
Informal:
* Es [day of the week]. (e.g., Es lunes.)
* Hoy es [date]. (e.g., Hoy es 15 de febrero.)
Example:
If today is Monday, February 15th, you could say:
* Hoy es lunes.
* Estamos a 15 de febrero.
* Es lunes.
* Hoy es 15 de febrero.
Remember:
* "Estamos a" is more formal than "Hoy es" when referring to the date.
* "Día" (day) is not necessary when answering.