Today, at 5:23 PM, the Autumnal Equinox occurs (in the southern hemisphere the spring equinox occurs). This is the day when the sun rises exactly in the east and sets exactly in the west. This is when the northern hemisphere starts to get colder as “the Sun crosses over the Earth’s equator on its way south” (more info from Universe in Motion). See the diagram for the Winter solstice to illustrate what it’ll look like in a few months.
I’ve also read that it starts at 14 minutes after the hour (instead of 23, but 23 seems to be most referred to).