Change default alert settings in Calendar on Mac
By default, events you create don’t have alerts associated with them. You can change alert settings, which are applied to new and existing events that you haven’t set an alert for.
In the Calendar app on your Mac, choose Calendar > Settings, then click Alerts.
Click the Account pop-up menu, then choose an account.
Your alert settings affect only this account.
Click the Events, All Day Events, and Birthdays pop-up menus, then choose an alert time.
The Birthdays pop-up menu is unavailable if you aren’t showing the Birthdays calendar. To show the Birthdays calendar, click General, then select “Show Birthdays calendar.”
If you don’t want these alert settings to apply to all of your devices, select “Use these default alerts on only this computer.”