High day means broad daylight or the day at its height. In John 19:31 it refers especially to the Passover Sabbath, which had special sanctity.
Definition
High day means the day at its height, or broad daylight, in contrast with the time when cattle are brought into their shelters for the night. The phrase may also denote a day of special religious importance.
Biblical Usage
In John 19:31, “high day” translates “great day” and refers to the Passover Sabbath, which had special sanctity. In Genesis 29:7, it denotes broad daylight or the height of the day.
Historical Bible Dictionaries
International Standard Bible Encyclopedia (ISBE)
<p>Is found in Ge 29:7 as a rendering of the Hebrew yom agadhol, literally, "great day." The Hebrew means the day at its height, broad daylight as contrasted with the time for getting the cattle to their sheds for the night (compare French grand jour). In Joh 19:31, "highday" renders megale hemera, literally, "great day," and refers to the Passover Sabbath--and therefore a Sabbath of special sanctity.</p>