There's no single best month - it depends on your route and where you are starting from. January departures catch summer in the Southern Hemisphere. September works for Southeast Asia's dry season. Avoid starting in June-August if budget matters - peak northern summer pricing affects transatlantic and transpacific fares.
There is no single best time, and anyone who tells you otherwise is oversimplifying. An RTW trip crosses hemispheres, time zones, and climate patterns. Summer in Sydney is winter in Stockholm. Monsoon season in Bangkok is peak season in Patagonia.
The real question is: what matters most to you? Good weather everywhere? Lower costs? Avoiding crowds?
If weather is your priority, you'll need to plan your route around seasons rather than picking a start date and hoping for the best. A January departure works well if you're heading west through Southeast Asia (dry season) and the South Pacific (summer), then up through Africa, and into Europe by late spring. A September departure catches the tail end of European summer, arrives in Southeast Asia for the cool dry season, and hits Australia and New Zealand in their spring.
Shoulder seasons (the weeks just before and after peak tourist season) are where experienced travelers find the best balance. Prices drop, crowds thin out, and the weather is usually still good. April to May and September to October are shoulder months in most of the Northern Hemisphere. The Southern Hemisphere flips: March to May and September to November.
For cost savings, avoid starting your trip during peak booking windows. Flights departing North America in June, July, and December are the most expensive. January, February, and October tend to offer the best fare availability.
Your AirTreks consultant will map out the seasonal patterns for your specific route and suggest timing adjustments. Sometimes shifting your departure by two weeks puts you in a different pricing tier or moves you from rainy season to dry season in a key destination. That kind of optimization is hard to do on your own, but it's something we do on every trip we build.