Tracking Contacts and Events - Teach Yourself Visually Complete Mac Pro - page 359

Click the Alerts tab.
If you are using multiple accounts, click the Account and then click the account to
configure.
Click the Events and then click the default alert time for your regular events.
Use these pop-up menus to modify the default alerts for all-day events and birth-
days.
Click Close ( ).
Each time you create a new event, Calendar automatically sets the default alert.