Nos pasamos vs pasamos

Question

I saw a sentence "Nos pasamos El día" it means we spent the day, but "Pasamos el día" means the same thing as well. I always get confused what's the reason behind using "NOS" in the first sentence.

Answer

Hola Harsh,

I would use “Nos pasamos el día” to say ”We spent the day with each other.” And “Pasamos el día” to say simply “we spent the day …” The former is making a reciprocal form of the verb to emphasise the each other aspect. Be careful with the verb Pasar because there are other constructions with “pasarse,” “pasarse a” and “pasársela” which have very specific meanings which I don’t think apply here though I am open to correction. Have a look in WordReference on-line dictionary and you will see. There is also a lesson on these on the course.

When I am confirming an appointment with my Spanish on-line teacher, I often sign off with “nos vemos entonces” meaning “see you then” but literally “we will see each other then.” I hope that helps. John

Answer

Hola Harsh 
I can see John's perception of that "nos" as a reciprocal mark but reciprocity I think comes with verbs and sentences where two people are seen as the objects of the verb, for example: 
nos besamos (yo te beso a ti y tú me besas a mí) 
we kiss (each other)
ellos no se entendían (el no la entendía a ella y ella no lo entendía a él) 
they didn't understand each other.
With "nos pasamos el día... " this "nos" is simply emphatic (se aspectual) to give that sense of "more involvement in the action":
Me pasé 3 horas en internet buscando la información.
I spent 3 hours on internet searching for the information.
Nos pasamos la noche viendo películas en Netflix.
We spent the night watching films in Netflix. (no reciprocity here at all) 
Se pasa todo el año en el huerto.
He spends the whole year in the allotment.
I hope it clarifies it.
Saluditos