To add a trigger date for automation in Tern, follow these steps: 1. Navigate to your Library and select Automations. 2. Create a new automation or edit an existing one. 3. Choose your trigger 'event' and set the 'timing'. 4. Determine when the action will occur based on your chosen timing. 5. Create your action, such as an email or task. 6. Ensure the automation is set to 'active' when applied. The trigger date derives from the event you select, like trip creation or start date.