On the equinoxes the angle of the sun is directly overhead at the equator.
On the solstices the sun is directly overhead a middle latitude in either
the Northern or Southern Hemisphere, called Tropic of Cancer and
Tropic of Capricorn, respectively.

When it is overhead in one hemisphere, it never gets high in the sky
all day in the other. Thus, the angle of the sun's rays causes seasons to occur.

The seasons are the reverse in the Southern Hemisphere of those in the
North. For example, in the Southern Hemisphere Winter Solstice is on
or near June 21st. The illustrations on these cards reflect the Northern
Hemisphere resident's point of view.

These are probably the most ancient holidays celebrated on Earth
because they are observable seasonal events that have been happening
since long before people were around to watch.