A beverage consisting of the juice of apples roasted over spiced ale. A great day for this drink was the feast of the apple-gathering, called in Irish la mas ubhal, pronounced “lammas ool,” and corrupted into “lamb's wool.”
“The pulpe of the rosted apples, in number foure or five ... mixed in a wine quart of faire water, laboured together until it come to be as apples and ale, which we call lambes wool.” —Johnson's Gerard, p. 1460.
Lambert's Day (St.), September 17th. St. Landebert or Lambert, a native of Maestricht, lived in the seventh century.
Be ready, as your lives shall answer it, At Coventry, upon St. Lambert's day.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894