summer solstice

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. In the Northern Hemisphere, the solstice that occurs on or about June 21.

Wiktionary

1. n. astronomy The moment when the Earth is in that point of its orbit where the northern or southern hemisphere is most inclined toward the sun.

WordNet 3.0

1. n. June 21, when the sun is at its northernmost point