Month names collection / metastudy

[Koen G]

However, this is good news for us because it helps us place the ŷ in a very specific context.

Yes! That's what I meant before. It would explain a lot, like why the accent is only on one "may" and not the other: if we are correct, it would be imposed by grammarians and not essential in any way. It just means "we once pronounced this as a diphthong but now not anymore". 

I agree that if this is indeed what we are seeing in the VM, it does limit our timeframe to a few decades in the middle of the 16th century.

There is still the option that the month names writer put the accent there as a fluke. But it would align with the idea of marking monophthongization. It would be interesting to know if this was actually used much outside of grammarians' works.