Conversation so far:
during which two months (equinoxes) do all locations on earth experience 12 hours of daylight
During the March 21 or 22 equinox and the September 22 or 23 equinox, every location on our Earth experiences 12 hours of daylight and 12 hours of darkness.

Question: when is the longest day
Self-contained rewrite: when is the longest day of the year