Wait, why are both ces and ges OK?
Ces is the base word. It used to be "Mi ges i" but now a lot of people drop the "Mi" but the mutation remains.
What does ‘mi’ mean again?
Nothing; it's just a grammatical particle.
But what does it refer to?
It marks a sentence as affirmative (i.e. not negative and not a question).
Interesting that there's a seperate word for last night. Would 'ddoe nos' be okay too, or is that a bit odd?