We'll start by picking a 2 month window. after which we'll try to find a week that will work for as many as possible. Keep in mind it can snow any day of the year up there. bugs can be bad as i remember it(it's been a while). in april and october i would be prepared for winter like conditions. during high summer there will be tourists. If you have never been to crater lake though I can safely promise it will be like nothing else you have ever seen.