How the date is fixed
Gita Jayanti falls on the eleventh day of the bright half of the lunar month Margashirsha (Shukla Ekadashi). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Gita Jayanti falls on the eleventh tithi of the bright half of Margashirsha, in November or December, which is also Mokshada Ekadashi. Tradition places Krishna's discourse to Arjuna at Kurukshetra on this day, at the start of the Mahabharata war. The Ekadashi rule applies, with the tithi present at sunrise, and the day is one of the two Ekadashis on which the fast is said to be complete in itself. Kurukshetra holds a week-long Gita Mahotsav around the date, and recitation of all eighteen chapters is the usual observance.
How it is observed
Recitation of the Bhagavad Gita, the Ekadashi fast, and public readings at temples and at Kurukshetra.