Spring, that means Sept and December are great times to go and not as crowded as school holiday. Winter is also good, often cold(light dusting of snow) with brilliant blue skies; some hotels and attractions will be closed, but the pubs are always open. Spring and autumn are becoming more popular
The worst time to visit the lake District is August, when it rains.
Weather does change rapidly in the Lake District, so you will need to pack the sunscreen. Some days that start out grey can often turn into beautiful bright sunshine. And the clouds certainly make it atmospheric. There's a local weather forecast called Weatherline that gives Met Office weather report and adds daily fell top assessor reports and photos during the winter.