When is the longest day in our country?

Summer solstice, of course. On the day of "Summer Solstice", the position where the sun points directly at the earth is the northernmost, pointing directly at the Tropic of Cancer, which is 23 30' north latitude, which is about the Guangning line in China. On this day, the solar altitude angle is the highest in all parts of the northern hemisphere, the longest day in a year, and the time when the northern hemisphere receives the most solar heat.