is it possible to check multiple tasks at once to complete multiple tasks at once, and vice versa, uncheck multiple completed tasks to uncomplete them again?
unfortunately, it's not possible right now, but thank you for your feedback, we appreciate it and will consider adding such feature in future development.
You're welcome to add a request to our Votebox: http://todoist.com/Vote/
For now, if you want to check off a task multiple times, you can use recurring date formats like: "every day", "every mon, fri at 15:00", "every weekday" etc. Tasks with such dates will not be moved to history after completion, but postponed to the next occurrence (until you use the "delete" option).
You can find the complete list of supported dates here: