The day of the year when the sun's energy was the strongest could be called the "Extreme Day"(Geng Day). "Polar Sun" referred to the day when the Earth received the most solar radiation during its revolution. This day usually appeared at the vernal equinox and autumnal equinox, respectively, corresponding to the polar day and polar night phenomena in the Earth's orbit. Because the sun was directly at the equator on this day, creatures on Earth usually felt the highest solar radiation intensity and temperature on this day. When writing a novel, the polar sun could be used as an important event or setting, such as a fierce battle or an important character being strengthened or mutated by solar radiation on this day. At the same time, the Polar Sun could also be used as a special day to commemorate or celebrate. For example, related activities or customs would appear on some festivals or celebrations.