Pack light cotton clothes for most of the year, but bring a warm jacket from December to February, when nights drop to about 7–8°C. April to June is extremely hot, near 40°C, and July and August bring the monsoon.
Delhi surprises people in winter. Most of the year it's hot, but from December to February nights drop to about 7–8°C, so a packable down jacket that squashes into a corner of your bag is the item first-timers wish they'd brought. Winter also brings the worst air, and well-fitting N95 masks are worth packing if you're sensitive to it.
For temples and gurdwaras, a light scarf covers your head or shoulders and doubles as a blanket on the plane. Add a travel sunscreen for the rest of the year, a compact umbrella for the July and August monsoon, a Type D adapter (most universal adapters won't fit), and a luggage scale for the gifts going home.
October, February and March are the most pleasant months. Winter also brings thick fog and the worst air pollution, so pack masks if you're sensitive to it. India uses Type C, D and M plugs at 230 V: bring a Type D adapter, because most universal adapters don't fit.
