Open your calendar in a full-screen view (/easy_calendar) and click on the pencil icon in the top right corner to edit it. Here, tick the checkbox "Holidays" and select as many country holidays from the list as you need to see, for example Germany, France, Poland...). When done, save the changes.
Looking at the calendar now, you should see holidays for each country you selected in the previous step. This way, you can check out which countries have holidays in which days, for example, in order to better coordinate teamwork or interaction with your foreign partners and clients.