Setting it up (HR)
In the holiday calendar, mark the optional days and set the calendar's quota (how many each person may pick a year). HR can see every pick and withdraw one if needed.
Picking (employees)
The Restricted holidays card on your Leave page shows the list, your quota and what is left. Pick a day and it counts as a holiday at once — attendance shows it as a restricted holiday and leave counting skips it. Withdraw a pick any time before the day.