These two seasons happen when Earth is halfway between Winter and Summer. Earth is the same distance from the Sun in Fall that it is in Spring. Earth's axis is still tilted towards the Sun, but Earth is tilted neither towards...

... Nor away from the sun. The Sun's rays, as a result, shine directly onto the equator. The equinoxes happen during Spring and Fall. These occur when the sun is directly over the equator and the day lengths are the same.

Spring and Fall