You need to create a custom Routine for the Turn On time and another Routine for the Turn Off time. Look for Routines in the Account, Settings, Assistant tab. It is NOT obvious how to do this so you might want to do some internet searches.
Unfortunately, when using a Routine, the Google Home Mini then verbally announces when it turns the light On or Off. There might be a better way to do this within the Google Home app, but I have not found it yet.