Are there any rules or guidelines for when to precede the name of a month with Mis and when not?
Usually you precede the name of the month with mis when you are saying things like “I am going to Spain in April” or “I was born in December”. When you talk about a specific date, you don’t use mis.
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Generally speaking, it should always be included, I think. But people are lazy (
) and sometimes miss it out. I would always use it in a full sentence like “Dw i’n mynd ym mis Chwefror”, but I were just giving the month I don’t think I would bother (“Pryd ti’n mynd?” “Chwefror”). An obvious exception to that would be Mawrth just for clarity (Tuesday or March)?
But that’s not an official answer - that’s just me 
Technically, the name of the month should always be preceded by mis, however, as Hendrik beat me to it to say, specific dates don’t need it.
“pryd wyt ti’n mynd ar wyliau?”
“10fed Awst”
It’s particularly important to keep the mis when talking about October - mis Hydref - to avoid it being confused with Autumn - yr hydref, and when talking about March - mis Mawrth - to avoid confusion with Tuesday - dydd Mawrth.
gee, Sara’s beaten me to that bit too! 
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You can always put mis in front, and there’s a bonus if you do, because then when you’re saying in… (month) you don’t need to put NM on all the different names - you just say ym mis… with the unmutated name. So instead of saying for example yng Ngorffennaf in July, or yn Nhachwedd in November, you just say ym mis Gorffennaf and ym mis Tachwedd. Brill!!
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Mis Genius 
Further to what Hendrik and Siaron were saying about mis with dates, both ways are fine and widely current. So for example the 20th of November is either
yr ugeinfed o Dachwedd
or
yr ugeinfed o fis Tachwedd
This is fun. 
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