Where did the celebration of Summer Solstice begin?

The ancient Indian Sanskrit texts known as the Rig Vedas contain the first mention of the earth revolving around the sun, this was approx. 1200 BCE. They were astronomy and astrology masters, so perhaps there....

In some traditions, Litha is a time at which there is a battle between light and dark. The Oak King is seen as the ruler of the year between winter solstice and summer solstice, and the Holly King from summer to winter. At each solstice they battle for power, and while the Oak King may be in charge of things at the beginning of June, by the end of Midsummer he is defeated by the Holly King.

When the celebration of the summer solstice began, is a matter of opinion.

Nobody knows. It's been celebrated in different forms by cultures all over the world.

It's very diverse since it is so widespread but Midsummer or Litha started around the 7th century with Saint Eligius.