This is the default "Calendar" software if you are using an Apple Mac, iPad or iPhone.
If there's no list of calendars on the left, click "Calendars" button in top left to show them.
Create a new calendar with "File -> New Calendar -> iCloud" (alternatively, use an existing calendar).
Add some events to your calendar, and give it a useful name like "Your Organisation - Public"
You can move events between calendars by clicking the small coloured square in the top right.
Click on the icon that looks like a wifi symbol next to the calendar name, and click on "Share calendar".
Copy the URL starting with "webcal://" it shows you, and email it to firstname.lastname@example.org.