IPA: //ˈnuːn.deɪ//
KK: /ˈnuːnˌdeɪ/
The time of day when the sun is at its highest point, typically around 12 o'clock in the afternoon.
We decided to have our picnic at noonday when the sun was shining bright.
Noonday → It is formed from "noon" (from Old English *non*, meaning the sixth hour of the day) and "day" (from Old English *dæg*, meaning a period of 24 hours). The word "noonday" refers to the time of day when the sun is at its highest point, typically around noon.
Think of the 'sixth hour' ('noon') when the sun is highest in the sky during the 'day' — that's what noonday means.