The Bihar region of India has three seasons:
March - June: The hot season
July - September: The monsoon season
October - February: The cool season (best time to visit)
The best time westerners to visit is November through February when the daytime temperatures are moderate and nighttime temperatures can be cool. Expect hot temperatures during the hot season, and rain, humidity, and heat during the monsoon season. Mosquito repellent is recommended - especially during the monsoon season.
Local Food and Water
Unfortunately, Bihar does not have western water sanitation standards. Before drinking, all water should be either boiled or purified with a portable water filter. Fruits are fine if the skin is peeled, but vegetables should be well cooked. It is recommended to eat at reputable eating establishments when dining in Bodhgaya. Stomach ailments are common for westerners, so bringing anti-diarrhea and intestinal medicines (tinidazole, metronidazole) is recommended. These medicines can be purchased in Bodhgaya.