You can add a public holiday to your calendar manually. Alternatively you can import them into your calendar, which is useful if you need to add multiple public holidays.
Click your name in the top right, then click Settings.
On the settings menu click Calendar, then click Public holidays.
Select the country you want to import public holidays for, e.g UK (England & Wales).
Under Public Holiday Groups, select a public holiday group e.g England and Wales.
Wait a few moments and then refresh your page. The imported public holidays now shows.
Repeat steps 1-6 for your other Public Holiday Group if you have one, but select a different country to import.