When I want to delete completed tasks at the end of month, I can't find a way, could you please help me. Thank you in advance.
if you're referring to the completed tasks listed at the bottom of your task list visible after clicking on the "Show completed tasks" list, then you can delete them all (in each project) using the "Delete completed" link on the bottom-right side.
If you're referring to the completion history in the upper-right corner, then unfortunately, it's not possible to remove it completely at once. We will consider adding such option in future development, though. You're welcome to add a feature request to our Votebox: http://todoist.com/Vote/
Thank you for your response. I was referring to the completion history in the upper-right corner. I add the vote box hope to see this feature in the near future.