From Wikipedia:
The summer solstice, also known as estival solstice or midsummer, occurs when one of the Earth's poles has its maximum tilt toward the Sun. Sun reaches its highest position in the sky. This day has the  the longest period of daylight and shortest night. 
Or defined in another way for northern hemisphere  - these are the moments when The Sun reaches the northernmost  point of its imaginary orbit, the so-called Tropics of cancer.

                                
Summer solstice on northern hemisphere usually comes on 20th or 21st June.
Northern hemisphere inhabitants celebrate Summer Solstice today 20th June 2020 - in middle Europe it is at 23.44 CEST (Central european summer time) = 21.44 UT.
           Summer solstice has been seen as a significant time of year in many cultures, since prehistory it was celebrated by different rituals and festivals. The life-giving power of the sun was from recently the object of worship, rituals and firmly connected with the human desire for life. The importance and strength of connection with nature testifies that solstice celebrations are celebrated almost all over Europe. It is astronomical beginning of summer on the northern hemisphere - though the meteorological beginning of summer start on 1st June already. 
It is said that the effect of everything that grows and blooms is amplified many times on the summer solstice.

                      Medicinal herbs in themselves preserve the power of The Sun. which is about the solstice at the peak of its forces.

                     It is one of the most magical days of the year. Primarily it is celebration of the spiritual power of The Sun.

                             Perhaps, for all ancient civilizations and cultures, the solstices were a cause for celebration

        Wishing you summer full of light and good energy

                                                                                                    Margaret