The seasons are reversed in the Southern Hemisphere because when the Northern Hemisphere is tilted toward the sun, the Southern Hemisphere is tilted away from it. Thus, when the Northern Hemisphere is receiving more direct sunlight and undergoing summer, the Southern Hemisphere is deep in the cold of winter.
The Earth's rotational axis is tilted 23.5 degrees away from perpendicular to the sun. For three months of the year, the Northern Hemisphere is tilted toward the sun, and for another three months, it is tilted away. For the remaining six months of the year, the tilt is neither toward nor away from the sun. While such a slight variation in orientation may seem insignificant, it is enough to drastically alter the temperature and weather experienced by each hemisphere. The change in angle affects how directly the sun's energy strikes the Earth, altering both the length of the day and the sun's apparent position in the sky.
The Northern Hemisphere's axis points most directly toward the sun around June 21 of each year, marking the summer solstice. It points furthest away around December 21, the winter solstice. The spring and autumn equinoxes mark the moment the Earth reaches a 90-degree orientation from the sun, occurring around March 21 and September 21 each year.