Summer Solstice technically happens when the part of earth is mostly inclined towards sun. Due to this inclination that part gets the longest day light in the year. And this marks the summer in that part, mostly days are longer than nights.
Therefore Summer solstice is an event that lasts for a day. But the effect of this inclination last for almost three months.
North of the equator i.e; Northern hemisphere gets days longer than 12 hours on June solstice. At the same time Southern Hemisphere gets days shorter than 12 hours.
Northern hemisphere gets days shorter than 12 hours on Winter Solstice. At the same time Southern Hemisphere gets days longer than 12 hours.