Choose the season by the half of Nikko you are going for. The shrine area near the stations is worth seeing in any season. Oku-Nikko, around Lake Chuzenji at about 1,300 meters, is far colder, its autumn color comes weeks earlier, and its one road up is where the traffic jams.
Autumn (October to November): match the date to the area
Color starts high and moves down. Nikko's tourism association forecasts Ryuzu Falls for early to mid-October, the Senjogahara marsh for mid to late October, Lake Chuzenji, Kegon Falls and the Irohazaka road for mid-October to early November, and the World Heritage shrines and temples for early to mid-November. That gap is useful: in early to mid-November you can see the shrine area at its peak without the climb to the lake.
The price of the lake at its peak is traffic. The Irohazaka road normally takes about 20 minutes by car, but the tourism association says it can take three hours on foliage weekends, especially from mid-October to early November, with the worst between 9:00 AM and 3:00 PM. Buses use the same road, and Tobu warns its buses to the lake can run more than two hours late, with no refunds. Going up before 7:00 AM on weekends, or after 3:00 PM on weekdays, is smoother; a night in Nikko makes the early start practical.
Summer (June to August): the cool half
Oku-Nikko is the summer escape. Japan Meteorological Agency averages for its lakeside station are about 19°C in August, against about 27°C in Tokyo. It is also wet: rainfall there is heaviest from June to September. The Lake Chuzenji sightseeing cruise runs from mid-April to 30 November.
Winter (December to February): the shrines, and the lake with care
The shrine area stays open through winter, with shorter hours: Toshogu closes at 4:00 PM from November to March, last entry 3:30 PM. At the lake, average temperatures stay below freezing from December to February and the cruise does not run, so a winter trip up is for snow scenery rather than the full Oku-Nikko day.
Spring (April to May)
Spring reaches the lake late: its April average is about 5°C, against about 14°C in Tokyo. Golden Week, from late April to early May, is the other period when the tourism association expects the Irohazaka road to jam, so treat it like a foliage weekend.
When should first-time visitors go?
For most first-time visitors, autumn is the best season, but pick the week by area: mid-October to early November for the lake and Oku-Nikko (ideally with a night in Nikko), early to mid-November for the shrines on a day trip. Outside autumn and Golden Week, a day trip can take in both areas with an early start.