iCloud User Guide
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Set an alert for a calendar event on iCloud.com
To receive an alert for a calendar event, you need to be signed in to iCloud.com when the alert goes off. If you are not signed in, you can still receive alerts on any device that has iCloud Calendar turned on.
Set an alert for an event
In Calendar on iCloud.com, double-click the event to open it (if it is not open already).
Do any of the following:
Set an alert: click the “alert” pop-up menu, then choose an option.
To remove an alert, choose None.
Set a second alert: click the “2nd alert” pop-up menu, then choose an option.
Click OK.
Set a default alert for new events
In Calendar on iCloud.com, click
in the bottom left-hand corner, then choose Preferences.
In the General pane, select “Add a default alert to new events & invitations” pop-up menu, then choose an option.
Click Save.
If you are not receiving alerts, make sure you have set the alert correctly and you are signed in to iCloud.com.