So you can't say them without the Mis?
No, except perhaps very casually once you have established that you are talking about months - several of the names have separate meanings:
and so on...
Oh! I didn't know that. Very interesting. Didn't realise they had other meanings
Same thing with the days of the week - they are are names of planets except for Sul, Llun, which are old words for 'Sun, Moon'.