Answer:
On June 21 each year (plus or minus a day depending on the cycle of leap years) the Sun reaches its northernmost declination of about 23.5 degrees. For people in the northern hemisphere, we experience longer days; earlier sunrises and later sunsets around that time. For people in the southern hemisphere, they experience fall and winter, with later sunrises and earlier sunsets.
On December 21 (approximately) the Sun gets as far south as it ever does, and we in the northern hemisphere get winter while the folks down south have spring and summer.