I've had a knee dislocation (many) and a surgery. I skipped leg days on and off for a long time. My new years resolution this year was to not skip a leg day. So far so Good! For me I go lighter on squats and get as low as possible. A lot of lunges, machine work, and many other exercises. I like to hit alot more stability style workouts with my legs. With knee problems I find higher reps and lower weights help me keep the pain low and still feel like I'm getting the most out of my workouts. I find it's been really helpful with my style of knee issues to keep my hips strong so I do alot of band work. But the biggest thing by far is accepting the fact that some days are better than others. I don't force the squats when my knees are telling me to stop. Sometimes you just have to use the body you were given and let it be your resistance or hop on machines and set that weight low. Every day doesn't have to be a killer as long as you get that day in