All you need to do is to open a task/email, click on the cog icon in the right corner and select one of options:
To reschedule a post for a specific day, select it from the calendar:
Note: you can check/edit rescheduled posts at any time - it will wait for you in the Pending folder:
You can postpone emails and tasks also from the Inbox list, by unfolding the cog icon: