On the Mac – open Calendar, then click File (menu) / New Calendar Subscription
Do not just click on the link – the calendar will not update with future dates as they are added if you do.
Instead go to Calendar / File / New Calendar Subscription
Then paste this in the calendar URL box and click subscribe:
If this calendar account is already on your phone, the nursery calendar should now appear on it it after a short delay. If not, click on iPhone instructions
Android – you will need to add this calendar to a Gmail account on a computer first, before it will appear on an Android phone.