Holidays take place in every country now and again and it is important that the employees are told about them ahead of time to avoid any confusion at the last minute.
The best and most efficient way to go about it is to send an email and let the employees know when and why the holiday will be taking place. It is important that everybody gets this email early so that they can organise or plan their work-load accordingly.
Start of the first paragraph with the information that the office will remain closed on the given date due to whichever special day.
In the second paragraph, let them know about the importance of the event and its significance. If it is a public holiday, do make a mention of that as well.
In the last paragraph, let them know about how the regular dealings for that particular day will be handled and if there is a need to work in advance for the day off. Wish them well for the day off.