Match wits with Cattubuttas the druid.
In the days of king Cú Roí, Cattubuttas the druid—said to be the wisest druid in Ireland, though he had not then a single gray hair in his beard—sat in a grove with his students, and this is what he said.
“As to foods, my children,” he told them, “the gods have denied us nothing, not even the flesh of the fleet-footed horse, noblest of animals.
“But know this also,” he added, raising a finger of admonishment: “that should it so happen that you do eat of horsemeat, it is thereafter geis upon you to enter into a chariot for the span of some twenty-seven days; for twenty-seven days thereafter, you may not enter one.
“Thrice nine days,” he told them again. “Remember it well, my warriors.”
So spoke Cattubuttas the druid to the young warriors in the days of Cú Roí the king.
And indeed, we still remember.
So: why 27?
In the martial society of Iron Age Ireland, such a prohibition—its memory preserved like a leaf in amber in Old Irish literature—would indeed lay heavy upon a warrior; it would, in effect, ban him from the field of battle for nearly a month's time.
The logic of the prohibition is not difficult to follow: it is, in effect, a breach of hospitality. Why, though, one wonders, specifically a period of twenty-seven days rather than, say, a full lunar month?
If Cattubuttas the wise, cat of battle, in his wisdom, knew, I for one do not.
Here's my guess, though: that it's numeric.