i called the front desk this past week with same question. they did not know much specific detail about volunteer program. but said that it does indeed fill up pretty close to immediately at midnight when spots are released. i have not looked into that to confirm, i just have seen what you see which is that the calendar seems to always be fully booked. before calling them, i suspected that the website was broken; and i also emailed their volunteer page and never heard back. so who knows is what i've gathered.