The timing of a visit to Sanmenxia depends entirely on whether you want to see the swans. The whooper swans arrive at Swan Lake from October onwards and peak in November and December, when the wetland can hold 30,000 to 60,000 birds. By late February most have departed northward. For the swans, November to January is the prime window.
For the Guo State Museum and Hangu Pass, the visit can be made year-round. Spring and autumn offer the most comfortable temperatures. Summer is hot but not unusually so; winter requires warm clothing, especially for Swan Lake visits early in the morning. The city hosts no major festivals that would motivate a specific timed visit independent of the swans.