After all, the northern hemisphere's Winter Solstice (December 22) -- the day on which the sun is at its most southern point in the ecliptic -- is the "Reason for the Season".
(the sun; is; at its most southern point in the ecliptic; T:the day) (the northern hemisphere's Winter Solstice; is; the "Reason for the Season)
the sun the northern hemisphere's Winter Solstice
is is
at its most southern point in the ecliptic T:the day
the "Reason for the Season