This high day is an actual astronomical event; in the Northern Hemisphere, it takes place this coming Friday, Sep 23, at 9:04 AM; I need to check to see the local time.

There are many names for this event; the September Equinox, The Autumnal Equinox, Alban Elfed (Modern Druid), Gouel Sant-Mikael (Breton), Goel Myghal (Manx), Goeldheys (Cornish), Gwyl Fihangel (Welsh), La Fheile Michil (Modern Irish), Mabon, Harvesthome or First Fruits (Wiccan), Michaelmas (Christian).

Since it is an actual astronomical event, this is what is happening:
The sun crosses the celestial equator and moves southward in the northern hemisphere during the September equinox. The location on the earth where the sun is directly overhead at solar noon is known as the subsolar point. The subsolar point occurs on the equator during the September equinox and  March equinox. At that time, the earth’s axis of rotation is perpendicular to the line connecting the centers of the earth and the sun. This is the time when many people believe that the earth experiences 12 hours of day and night. However, this is not exactly the case. (News to me! Apparently equal day and night is a metaphor, not a literal truth.)

Sources:
http://www.applewarrior.com/celticworld/celticyear/quarters.html

http://www.timeanddate.com/calendar/september-equinox.html