The Discovery Solar Plaza is both a solar calendar and a solar clock, which was laid out by students in concert with the construction and design teams that conceived it.
In the winter the sun is low in the sky and the sunspot from the oculus is just a sliver. During the year, as we get closer to summer and the sun moves higher in the sky, the sunspot travels southward and becomes closer in size to a full circle. On the two equinoxes in March and September, the sunspot travels in a straight line throughout the course of the day. When the sunspot is south of the equinox line, we are in the warm season. The longest day of the year is the summer solstice. On this day, the sunspot will travel along the curved line for a day before starting its trip back northward to the cold season.
The solar calendar is established using Daylight Saving Time, which encompasses the summer solstice and both equinoxes. Therefore, during the winter the solar calendar will read one hour later than our clocks.
You can watch the sun spot projected through the oculus pass across the central Equinox Line in this video from the Autumnal Equinox of 2016.