From the Ground

There is the same logic behind safe period and periodic holidays.

First, mark 52 working days with blue ball pen, or whatever color you like. 

Then, mark 8 holidays with black ball pen, or whatever color different from working day color. 

Next, highlight the holidays you really take.

Always an accident or something unexpected occurs before you leave office. (messy period?)

Now your holidays of the entire year are shown on the yearly calendar.

There will be at least 3 different colors on your calendar.  Compare your periodic holidays with your private calendar.   Isn't it the same logic as calculating safe period?

Once in a while, you'd like to change the shifts with your colleague; you have to prepare everything thoroughly before taking action.

我覺得算假期和算安全期差不多

先拿藍筆標出工作日

再拿黑筆標出週期性的假日

然後用瑩光筆畫出真正的假日

偶爾想要調班什麼的，還得先做好萬全準備（以免代班同事不知情）