Nooo, summer doesn't start today...! According to the old folklore calendar, Summer begins on Beltane (May 1st) and ends on Lughnassadh (August 1st), with the Summer Solstice midway between the two, marking MID-Summer. This makes more logical sense than suggesting that Summer begins on the day when the Sun's power begins to wane and the days grow shorter. Litha or the Summer Solstice is usually celebrated on June 21st, but varies somewhat from the 20th to the 23rd, depending upon the Earth's rotation around the Sun.
Blessed Litha, Happy Summer Solstice!!!