Gita Jayanti is the birthday of Bhagvad Gita,
It is celebrated on the Shukla Ekadasi, 11th day of the waxing moon of Margashirsha month (November-December).
The "Bhagavad Gita" was revealed to Arjuna by Sri Krishna in the battlefield of Kurukshetra (in present day Haryana, India). Krishna told Arjuna to act according to his dharma.
A summary of Bhagavad Gita.
No body in the universe can stop activity. Every individual keeps doing some thing or other activity. Every individual has the right to do a proper activity after due thinking, but results may not happen as expected as the rest of the world acts in its own way and it is not under the control of one man. So a man should not get attached too much to the results of any particular action. But he has to keep acting according to his knowledge of dharma. In the Bhagavadgita, Krishna described Jnana Yoga that gives knowledge to one to make sense of the world. Raja Yoga which is doing great things in this world which are not within the abilities of many people. Karma yoga is doing activities that one has to do in a natural way as he comes across in his life. Bhakti yoga is great devotion to God or Jnana and Raja yogis or even karma yogis. Bhakti yoga talks of willingness to follow, waiting for directions and implementing the directions like a karma yogi. Hanuman is considered a bhakta, even though he is a great Raja Yogi. He did many great adventures. But he did them on the directions of his object of devotion Shri Rama.