In the calendar list on the left, click the down-arrow button next to the appropriate calendar, then select Edit Notifications.
In the Event reminders section, select the reminder method from the drop-down menu and enter the corresponding reminder time (between one minute and four weeks). If you'd like to add additional default reminders, simply click Add another reminder.
Please note: when you make changes to the reminder settings for events on your calendar, the changes will only apply to you. Invited guests or those who have access to the events will receive reminders according to their own Google Calendar reminder settings.