At this time of year you can calculate the crowd by adding the quota + plus a few extra for those summitting and then heading down the main trail + a few extra day hikers. There will be more day travellers on the weekends, but it will never be as crowded as the main trail. I did a day hike on Labor Day, and I saw 3 or 4 tents at Upper Boy Scout Lake, and none below that.
Baldy and Gorgonio see more fluctuation in visitation between weekends and weekdays.
Baldy's crowd is to a large extent driven by the easier access afforded by the ski lift (weekends). Crowds drop off dramatically in mid-afternoon as hikers descend to meet the last chairlift. In late afternoon there are only a couple of people on the summit. Trail crowds are inversely proportional to the difficulty of whichever of the various trails that you use to get up there.
I did Vivian Creek on Mon., Aug. 28, and met about 4 overnight parties and virtually no day hikers.
And Langley is surely the most solitary of them all, with much of its traffic being sucked up by the more glamorous Whitney. I did Langley the day after Labor Day and met no one between New Army Pass and my return drop down to Cottonwood Lakes.
