The National Park Service visitor center is open year-round, though hours are reduced in winter. The site itself — the five historic buildings and the surrounding prairie — can always be seen from the outside, and the flat Kansas landscape means the light is often beautiful in the early morning and late afternoon.
The single best time to visit is the last weekend of July, when Nicodemus holds its Annual Homecoming. Descendants of the original settlers return from across the country, and the town becomes briefly, vividly alive with music, food, and reunion. It has been held continuously since the late nineteenth century, which makes it one of the longest-running such events in the country. If you are planning a trip, build it around the Homecoming — the contrast between the quiet town you arrive in and the community it becomes that weekend is itself a lesson in what Nicodemus means to those whose families helped build it.
Summer on the Kansas plains is hot (regularly above 35°C / 95°F) and can be windy. Spring and early autumn are more comfortable for walking the site. Winter visits are peaceful but spare — some interpretive services may be limited.