Translation:We have not seen each other since Wednesday.
I am sure there is a faaaaar better explanation to this, but the first thing that pops into my head is that:
Nós = we
Nos = us (+ourselves/each other)
Let me know if you find an exception, but I think this is the simple way of explaining the difference.
(edit: thanks, paulenrique, I added your suggestions to the explanation!)
One's the subject, one's an object pronoun. That's the general idea, but there's a bit more to it.
"nós" is used as the subject of a sentence and after prepositions (para nós; to us, for us), whereas "nos" is used as the object of an action. "Eles nos usam para aprender portugues"; "They use us to learn Portuguese"
Do take what I say with a grain of salt. I'm still learning and it's perfectly possible that something I said is completely wrong.
It's not a stupid question at all. I can't give you the rule, since I'm learning this from the ground up like everyone else around here, but I've seen these enough to recognize a certain pattern. "Desde" is the trick to this sentence. It carries the temporal weight. Just like in English, if you use the word "since" then you are referencing to some past event or idea. Once you have that in place, the tense on the verb is much less important. In English we happen to use the present tense "have", but the meaning exists in Portuguese without it. The literal translation "We not each other see since Wednesday" is the proper grammar in Portuguese. Through tradition we say in English, "We have not seen..." In both cases, the word desde/since is the important reference.
(Note: Do not confuse this use of "since" with a common Americanism. Most of us often use "since" to mean "because," but that isn't correct (in formal English). Ex: "Since you know how to do this, I won't explain," would not translate to "Desde você..." but rather "Porque você..." EDIT: Or "Já que voce..." is okay, too. Go figure!)