Yoga is a 5,000-year-old physical, mental and spiritual practice having its origin in India, which aims to transform both body and mind. On December 11 in 2014, the United Nations General Assembly declared June 21st as the International Day of Yoga. The declaration came after the call for the adoption of June 21st as International Day of Yoga by Hon’ble Indian Prime Minister, Mr. Narendra Modi during his address to UN General Assembly on September 27, 2014 wherein he stated: "Yoga is an invaluable gift of India's ancient tradition . It embodies unity of mind and body; thought and action; restraint and fulfillment; harmony between man and nature; a holistic approach to health and well-being. It is not about exercise but to discover the sense of oneness with yourself, the world and the nature. " In suggesting June 21, which is the Summer Solstice, as the International Day of Yoga, Mr. Narendra Modi had said that, "the date is the longest day of the year in the Northern Hemisphere and has special significance in many parts of the world. "
Yogi and mystic, Sadhguru notes the importance of this day in the yogic tradition: “On the day of the summer solstice, Adiyogi [the first yogi] turned south and first set his eyes on the Saptarishis or Seven Sages, who were his first disciples to carry the science of yoga to many parts of the world. It is wonderful that June 21 marks this momentous event in the history of humanity.”
Source & Credit : http://idayofyoga.org/index.php?route=information/information&information_id=4