The summer months in Europe are June, July and August. This is because the entire continent of Europe is located in the Northern Hemisphere, above the equator. The summer months are the months where the Northern Hemisphere is oriented more toward the sun.
The earth tilts at 23.5 degrees on its axis. When the earth is oriented so the Northern Hemisphere is tilted more towards the sun, the sun stays above the horizon for a longer period of time and appears higher in the sky. At this time, the rays from the sun hit the ground more directly than when the Northern Hemisphere is tilted away. Summer typically begins around June 21 and ends around September 22.