On June 21, the summer solstice occurs in the northern hemisphere and the winter solstice occurs in the southern hemisphere. The tilt of the Earth's axis causes different parts of the planet to receive different amounts of sunlight at different times of the year. During the summer solstice, the northern hemisphere is tilted toward the Sun, while the southern hemisphere is tilted away from it, causing opposite seasons.
New York is located in the northern hemisphere, so it experiences summer on June 21. Jakarta, on the other hand, is located in the southern hemisphere, so it experiences winter on the same date.