Summer Solstice marks the longest day and shortest night in the Northern Hemisphere. In the Southern Hemisphere it is called the Winter Solstice because there it is shortest day and longest night.
The time of the Solstice is the same throughout the world. It the moment when the Sun reaches it highest point north in the sky and just for that moment it doesn’t move north or south, farther up or down. Just for that moment. Pretty cool?