It has nothing to do with a trigger. It's set on comment create. I guess you could flip a comment to a system comment with that trigger if you wanted, but it's not how you'd schedule creating comments. To schedule creating comments you'd just call your auto action from a CRON task to run it regularly on whatever schedule you want. Example as follows.
Global
Triggers: None
Type: CB Activity
User: Self
Access: Everybody
Action
Mode: Comment
Method: Create
Published: 1
Asset: activity.ACTIVITY_ID_HERE
Message: HTML_HERE
Owner: OWNING_USER_ID_HERE
System: 1
Load By: None
This will create a comment on the activity entry with an id matching ACTIVITY_ID_HERE and render it as a system comment supporting HTML.