iCloud 
Set an alert for events
An alert is a message reminding you of an upcoming event deadline.
To receive an alert from the iCloud Calendar web app, you need to be signed in to iCloud.com when the alert goes off. If you’re not signed in to iCloud.com, you can still receive alerts on any device that has the iCloud Calendar feature turned on.
Set an alert for an event
Double-click the event to which you want to add an alert.
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Choose an option from the “alert” pop-up menu.
To have the alert appear when the event begins, choose “On date of.”
To add a second alert, choose an option from the “2nd alert” pop-up menu.
Click OK.
Remove an alert from an event
Open the event with the alert you want to remove, choose None from the “alert” pop-up menu, then click OK.
If you’re not receiving alerts
Check the following:
Double-click the event to make sure you set an alert for that item.
If you want to receive the alert from iCloud Calendar, make sure you’re signed in to iCloud.com when the alert is set to go off.