Simple, type "settings", head to the link shared with you.

Head to Vacation Management-->Calendar-->Holidays to create holidays for your team. 

If you have people working in different countries or offices, you can also create office and country specific holidays. This way people in a specific country/office will only see holidays specific to that country/office. 

If you have multiple offices across each country and all offices have different holidays then choose the office while creating the holiday. For more details please read more about multi office holidays

Once created, your team can lookup upcoming holidays by typing 'holidays' and plan their vacations accordingly.