Thanks! Also, how do I see you answering questions so often... IN SO MANY DISCUSSIONS??? (By that I mean in different languages in Duo).
What does ym really mean? Is it a compound of words, like im in German? Why don't you say "Y mis Ebrill"?
Ym, yn, yng mean exactly the same thing: just like a, an in English, their shape depends on the shape of the following word.
The basic word is yn, but it turns into ym before m (whether the word originally had the m or whether it was a b or p that the yn turned into m or mh) and into yng before ng (as the result of a nasal mutation of yn on g, c into ng, ngh - I don't think any word starts with ng in its unmutated base state).
It's for ease of pronunciation -- pretty much the same principle by which in- in Latin words turns into ing- (pronounced, but not spelled that way) in words such as incorrect and into im- (also in spelling) in impolite, imbibe. Except that Welsh doesn't do the irrelevant, illogical thing and turn yn into yr, yl before r-, l- :)
And I am as blind as a bat today! Thank you for your patience with this near-sighted fool. Of course, now I see it quite clearly, but earlier I did a double-take and checked it more than once. So, it is off to the optometrist for me!