Yin is a very slow paced kind of Yoga. It's practiced mostly sitting, and every pose is held from 3 up to 5 minutes.
It works primarily on our joints and the non-elastic tissues of our body, such as bones, tendons and ligaments.
Even though it's very slow and with no apparent movement, Yin Yoga doesn't have to be cofused with "restorative" Yoga. While the letter is meant to put as less stress as possible on our body, Yin Yoga is meant to push our bondaries and challange our comfort zone.
It's direct work on our nervous system makes it a very calming practice.
If you have the freedom of choosing when to practice it, I would say that you would get the maximum benefits for your joints in the morning, when the muscles are still cold and rather stiff. In the evening it's wonderful to shrug off the tensions of the day and to prepare yourself for a deep, replenishing night sleep.