Wherefore art thou, aberil?

[nablator]

But how about the late middle ages, and perhaps regional variation? (Real question - I don't know).

I don't know a medieval occitan dictionary.

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Here they have abriel, abril, avril:
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Etymological dictionaries have most of the ancient variants:
You are not allowed to view links. Register or Login to view. --> abril, abriol (1475, Fazy)