Around the fall equinox and spring equinox the temperatures are more moderate because the Earth is at a position where it is neither in its closest, not furthest position from the Sun, the speed of the Earth slightly decreases as the spring equinox is nearing, so it starts to warm up, and slightly accelerates when the autumn equinox is nearing, so it starts to cool off. So to put it simple, these are the periods where the Earth is not in a position and speed where the Earth experiences extremes, but instead these are the transitional periods, so they are much more moderate than the extremes.