Plural form of heydey; multiple archaic instances of heydays or periods of greatest success (using the alternate spelling).
From heydey (alternate spelling of heyday) + -s for plural, preserving the non-standard spelling in its inflected form.
Finding multiple plural forms of the same word (heydeys vs heydays) in historical texts is like finding fossils—it shows how language evolves and how standardization gradually smooths out variation.
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