As soon as you can start casting is a good time to start. The northern end is better earlier and the southern end is better later. It's not too early to start now and it will usually be over by the beginning of May. The first couple of weeks of May are great for spawning Muskie though.
Walleye can be caught during the day and some are caught during the night but the best is wading and casting a jig tipped with minnow up until just before dark and then switching to a rapala for an hour or so after dark. This is the best period in my experience. Drifting crawlers during the day can work pretty well early too. I found I could catch a good many fish just wading for the last few hours of daylight and I didn't need to mess with a boat which made it more easier for me. I lived beside the lower end of the lake for years. I have property there yet and really want to move back up there but money and the economy are holding me from getting it accomplished.
They can be caught at anytime obviously but this has been the hot period for me. Others methods and times may vary.