Ranthambore is open from October to June; the park closes completely during the monsoon (July to September) when the roads flood and vegetation is too dense for good wildlife sightings.
October to February is the best time overall. The weather is cool (nights can be cold — bring a layer for dawn safaris), the vegetation is not too thick, and tigers are active during daylight hours. December and January are the coolest months and the most comfortable for long days outdoors.
March to May offers outstanding tiger sightings as the vegetation dries out and animals congregate around the remaining water sources. By May, temperatures reach 40°C and afternoon safaris are uncomfortable, but the morning slots are golden — literally, in light terms, and for tiger activity.
June is the last month before closure: hot, dusty, and fewer visitors. Occasionally excellent for determined wildlife photographers who don't mind the heat.
Book safari permits online through the Rajasthan Forest Department website well in advance for peak season (November to March). Last-minute permits sometimes become available but cannot be counted on.