Solstice

According to wikipaedia, a solstice is an astronomical event that occurs twice each year, when the tilt of the Earth's axis is most inclined toward or away from the Sun, causing the Sun's apparent position in the sky to reach its northernmost or southernmost extreme.

This year this event will happen at 05:45 on 21st June

Thats (part) of the science of it. The solstice also has many other cultural and mythological interpretations, but I am going to stick with the simplest. The longest day...the shortest night...when we dare to go where we never went before.