Noonday means happening or appearing in the middle part of the day. It was hot, nearly 90 degrees in the noonday sun. = midday. happening or appearing at noon = midday
Literally, a demon thought to be active at the noon hour which inclines its victims (usually monastics) to restlessness, excitability and inattention to one's duties. Used as a synonym for acedia (Gk. ακηδία)