David W. Maxwell writes:
From where in the world does the word heyday come? As in, "The crooked politicians were were in their heyday during the Tammany Hall period." Is it possibly an African-Americanism?
Pretty unlikely, given that the word first appears in the late sixteenth century, not a time of substantial African-American contribution to the English language.
The main meaning of heyday is 'the stage or period of greatest vigor, strength, success, etc.; prime'. This particular nuance dates from the mid-eighteenth century, in Smollett. This is derived from the earlier (late sixteenth century), and now archaic, sense 'high spirits' ("At your age,/The heyday in the blood is tame"--Shakespeare, Hamlet).
The attentive observer may note that this original sense does not seem to have anything to do with 'day', and in fact the second element of heyday is thought to be a different word. (Association with the familiar sense of day surely has influenced the later 'period of vigor' sense, though.)
Two other words may contribute an influence here. There is an earlier heyday, in this case an interjection used as an exclamation of cheerfulness, surprise, wonder, etc. Some examples: "Hey-day, why so nimble, and whither so fast, said she?" (Richardson, Pamela); "Heyday! Pray, what does he want with me?" (Dickens, Martin Chuzzlewit). This word is first recorded in the early sixteenth century, in various forms (heyday, heyda, hoida, hoighdagh, and others); it is probably an extended form of the common exclamation hey, and may be related to similar words in other Germanic languages.
The other term is the twelfth-century high day, literally 'a church festival or other holy day', and later (sixteenth century) 'full daylight; a time of day when the sun is in the sky'. Scholars have made various conjectures about the relationships between this high day, the earlier heyday!, and our word--that our heyday derives from either sense of high day, influenced by the exclamation heyday!, or that high day is a coincidence. It appears that heyday! has had an influence on our heyday, but it may not be possible to be more specific than that.