It's not uncommon that certain events are relevant for you content processes but aren't an actual Social Post, Campaign or Story. Typical examples are public hollidays or physical events, but anything goes.
Step 1: Navigate to your workspace's calendar.
Step 2: Click on the day the event takes place and select Add an Event
Step 3: Give your event a title and optional description. You can also make it a multi-day event or change the starting date. Uncheck All day to also set a specific time. Once your happy press save.
🎊 You're done.