I think the easiest would be to use a mains timer like this **broken link removed**
Its quite versatile, gives a number of program cycles per day, or day of the week.
I have used quite a few of these, and they are quite reliable.
At the output of the timer, just connect your 12 volt power supply that drives the motor.
Note that this is a 240 volt version, in case you are in a 120 volt country. Also, they come in 2 versions; dry contacts (no mains on the output, just the relay contacts bought out), and switched mains output.
Regards.