there isn't a really good way to do it if it will need updated - the admin could use PowerShell to inject events into everyone's calendar, but it's not necessarily easy. (I'm not sure if it works with office 365 since you can't put the pst on the exchange box).
My recommendation is either a public folder (or group) calendar or a calendar in a mailbox - send a sharing invite to everyone and when they accept it will be in their outlook. (You can share subfolders or non-default calendars using right click > share.) Any of these would be easy to update as needed.
if it's a one-and-done thing (for all day events), you could create a custom hol file and send it out - users would just need to open it and click Add to calendar. But it's not something you'd probably want to do to add events on a regular basis - and it won't move or remove events, just add new.
We may eventually have the ability to create "interesting calendars" for tenants, which would be perfect for your scenario. I have no idea if or when though.
