Unless you have a contract or local law states otherwise, your employer is in charge of your schedule and can compel you to work whenever they deem necessary.
No US employer can ever compel you to work for free. ALL work, without exception, must be paid. The employer can change your schedule, but must pay for your work.
the employer alone sets and changes your schedule, and must pay you for all days worked.
When an employer posts a work schedule depends on the length of the schedule. This could be at the start of the week, start of every two weeks, or the start of the month.
An employer can change your schedule for any reason, and you work the schedule set by the employer.
I think it is possible. Employees directed by superiors to travel are in work status, not a vacation during which they are free NOT to work. As long as employer requires work, and exercises control over employee, it is paid work.
IF you mean can it cancel your vacation and schedule you to work, then yes.
No federal law states that you are to be given 24 hour notice prior to a change in your work schedule.
they can make sure their personal lives do not interfere with their work
You can't ask a prospective employer if you can work your ideal schedule if you don't know what it is.
You can't ask a prospective employer if you can work your ideal schedule if you don't know what it is.
You can't ask a prospective employer if you can work your ideal schedule if you don't know what it is.
It is important to have good communication with your employer. Make sure to speak with the employer about your concerns and explain your expectation of notice of any changes to the schedule. The employer wants to keep employees happy, so hopefully they will make an effort to correct this problem in the future. Give them the benefit of the doubt this time and assume there was a misunderstanding.