How many days in advance should I book international flights for December travel?

For December — India's most brutal fare season — our working rule is 10 to 12 weeks out, so late September to early October, and earlier still if your dates pin to Christmas–New Year week, when we'd say August. The pattern every year: fares sit stable through summer, begin their climb around Diwali as the whole country books at once, and hit silly numbers by mid-November. Three tactical edges: fly the first half of December and return before the 22nd to dodge the festive doubling entirely; check the new routes (Navi Mumbai's international services, the fresh Gulf directs from smaller metros) where launch pricing undercuts the established corridors; and for Southeast Asia, price the open-jaw (into one city, out of another) — it frequently costs no more and saves a travel day. Set fare alerts now regardless; December forgives early birds and punishes everyone else.