Timing your visit to Crater Lake depends on which activities you most enjoy. Summer, obviously, is the most popular time to visit because the park is most accessible. In addition to viewing the lake from the rim or by boat in the caldera, there are opportunities for hiking, camping and learning about the lake’s history and unique ecology.
In wintertime, outdoor activities turn to cross-country skiing and snowshoeing (free, guided hikes are offered by volunteer rangers generally from November to April). Crater Lake officials always say winter is the park’s dominant season, considering that it is snow-shrouded most years from October to May.
I would recommend visiting both times of year.