Might I Inquire Where The Word Mehefin Comes From? It's Not A Cognate Of The English, Perhaps It Comes From Latin? My Latin Isn't That Good, So I Don't Know.
The me- perhaps comes from the same root as 'mid-' and the Latin 'medius', words with a common Indo-European root. The -hef- is perhaps cognate with haf (summer).
In the agricultural cycle, it was the time of year by when the animals and their spring-born young were moved up on to the higher land again, to their usual cynefin (heft), a word with similar elements.