I was just wondering how far in advance is an employer required to post schedules?

My employer post them on Friday- for the following week. It is driving us nuts because we cannot make appts & plan vacations cause we never know what we are working. Yes we have all signed contracts w/set schedules but we are so short (not short- tight) that we are all working alot of xtra hours. We have reached a point where they are no longer asking us- just scheduling us. I changed positions from 40 hr flex to a set schedule to avoid this problem. Have yet to work my new schedule, they just schedule me where ever & now pay me a dollar less an hour!

So if someone knows if they are legally required to post a schedule like 2 weeks in advance or some time frame it would be appreciated.