Final answer:
New York City experiences its longest day of the year on June 21, the summer solstice, with around 15 hours of daylight due to the Earth's axial tilt. This contrasts with the equinoxes with equal day and night, and December 21, the winter solstice, with the shortest day.
Step-by-step explanation:
New York City has its greatest length of daylight on June 21. This day is known as the summer solstice in the Northern Hemisphere and is recognized for having the longest day and the shortest night of the year. The reason behind this fact is that on June 21, the Earth's North Pole is tilted towards the Sun, and the Sun's path in the sky passes north of east and sets north of west, resulting in approximately 15 hours of daylight above the horizon in the United States.
After the summer solstice, days start to get shorter again as the sunrise and sunset points migrate southward. The other significant days that relate to daylight length are the equinoxes on March 21 and September 21, when day and night are roughly equal in length, and December 21, which is the winter solstice with the least amount of daylight and longest night.