Go to the People page and click on Calendar which you can find on the right sidebar.
Here you can see all upcoming birthdays and holidays. Also, you're able to add new holidays or even make Saturday a working day!
In order to do this, click on New holiday.
Provide the name and start/end dates. If you want this holiday to be a working day (e.g. you need to create a working Saturday), check is workday. If you check notify, you will be informed about the upcoming holiday. After you click Create you will be redirected to the all holidays page. You can modify or even delete any of them.
When you go to Calendar view again, you will notice your newly created holiday already visible: