The astronomical beginning for the summer season will happen officially at 6:28 AM CDT today, June 21st when the Summer Solstice takes place.
The Summer Solstice occurs when the path of the sun reaches its northern most point in the northern sky. As this name directly relates to the season in the northern hemisphere, this date may be known as the Winter Solstice for those in the southern hemisphere to correspond with the season of winter beginning. The Summer Solstice is also known as the June Solstice, with it occurring usually near June 21st.
The date of the Summer Solstice corresponds to the longest day of the year for the northern hemisphere (and the shortest day of the year for the southern hemisphere). The sun is centered on the Tropic of Cancer (23.5 degrees North), leading to the areas above the Arctic Circle (66.5 degrees North) to receive a full 24 hours of sunlight and for areas below the Antarctic Circle (66.5 degrees South) to have a full 24 hours of darkness. Here in southern Wisconsin, the sun will rise around 5:15 AM CDT and set around 8:37 PM CDT (give or take a few minutes depending on exact location in the state). To get the exact time at your house, please visit the Naval Observatory page.