So if you think of the seasons as a sine wave, the first day of spring represents the zero crossing point where the phase expands until it reaches its peak first day of summer at which the phase starts to collapse until it reaches the zero crossing point and starts to go negative. We call that autumn. The wave expands negatively until it reaches winter at which point it starts to collapse. The root mean square of the phase in summer is between May 21st and July 21st.

All because the earth is round and rotates at an angle to the sun. 🌎