Lillooet is in a rain shadow, which gives it some of the most extreme weather in BC: very hot summers (regularly above 35°C), cold winters with snow, and an annual precipitation that's surprisingly low for such a mountainous area. This continental climate is what creates the distinctive dry canyon landscape.
Summer (June to September) is the best time for outdoor activities — hiking, river swimming, and the salmon fishery at Bridge River Rapids, which peaks in August and September. Late May and early June are excellent too, with wildflowers covering the benchlands. Winter can be quite cold but the town sees occasional days of clear sunshine that are beautiful.
The Duffey Lake Road connecting Lillooet to Whistler is only reliably open in summer and early autumn — from roughly late May to late October, depending on snowpack. Driving it in ideal conditions is one of the great scenic drives in BC.