The road to Tawang via Dirang is open all year but can be blocked by snow at Sela Pass in mid-winter. Best months for driving and clear views are October–November and March–May.
Winter (December–February) is genuinely cold — temperatures drop below freezing — but it is also when the black-necked cranes are in Sangti Valley, and when the mountains around Tawang are spectacular under snow. Pack accordingly.
Monsoon (June–August) brings landslides on the road up from Tezpur. Avoid if you can.