Retreats in Bodh Gaya have been held annually since 1975, and the Dhamma meetings in Sarnath since 1999. Both are held on a donation basis.
Registration for a retreat in Bodh Gaya by email to:
Further information about the events in India is available at www.opendharma.org
Travel to Bodhgaya is usually via Gaya, which is 13 km away. Gaya is on the main railway line between Delhi and Kolkata (from Delhi at least 18 hours to Gaya, from Kolkata at least 8 hours).
You can also fly from Delhi to Patna and from there travel by bus to Gaya in at least 4 hours. A flight from Kolkata to Patna will almost certainly not be worthwhile.
When entering via Delhi, it is therefore worth considering booking a connecting flight to Patna, but this does not necessarily work in India.