Today is the First Day of Spring - when the Vernal Equinox occurs in the Northern Hemisphere.
In Tanzania, and the southern Hemisphere, it is the First Day of Autumn (the Autumnal Equinox). This means that this summer when we are on our trip to Tanzania it will actually be during their winter.
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator.
The name is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long.