The spring is much the same as the winter at Crater Lake. While the temperatures warm as the season goes on, the deep blanket of snow remains, not reliably melting out until well into the summer months.
The summer is short in the park, and is when almost all visitors arrive. Snow can remain until August, and all of Rim Drive does not open until well into July in some years.
The fall starts out as a great and relatively quiet time to visit, until snows start to close things down, usually in October. Snow should be expected any time in September as well.
The winter brings some of the heaviest snows in the United States to the park. The road to Rim Village is normally kept open, but all of Rim Drive and the North Entrance Road are long since buried in snow. Most facilities are closed, and a vehicle (and driver) well experienced in winter driving are recommended for a visit.