This is what we have in our handbook:

You may usually take your personal days whenever you choose after one year, subject to the following restrictions. Request your personal days at least one week in advance. We ask that no more than one person per department is off work simultaneously. Occasionally, exceptions are made. [Employer] reserves the right to refuse an employee's Personal Day dates, if in [Employer's] sole judgment scheduling the Personal Days at the time sought would disrupt anticipated customer jobs during that period of time. Contact your supervisor for approval of your Personal Days dates. Personal days are scheduled in the order requests are received. The above notwithstanding, employees aren’t permitted to take an unlimited number of days off work.