To make your tasks easy to manage, only enter dates if you have to. Don't set a due date just to bring a task to your attention. You can use Priority, Star or Next Action for that.
Generally speaking you should set a due date on a task if you have promised someone else you will do it by then, or if you have promised yourself.
To set a date, click on it. A date-picker window pops up. You can browse through the calendar, or use one of the handy shortcuts.
You can also set start times or due times for your tasks. To set a time, click on it and select the time you want.

If you want to do a task regularly, for example "Clean the car every Sunday", set the task to Repeat Regularly
, with a due date of next Sunday, and set the frequency to Weekly. You can do this in the Task List, or in the Task Properties window.
Some tasks you might plan to do every Sunday, but maybe you don't actually do it until Wednesday. When do you want to do it next time? If you want to stick to your schedule, then leave it at Repeat Regularly. But if you want to wait a whole week before doing it again, change your repeat setting to Regenerate on Completion. How dirty can a car get in four days?
The icon for Regenerate on Completion has a red check-mark running through it
. Clicking on the icon changes the repeat setting.