To maximize the benefits of Yin Yoga and increase your flexibility, it is best to practice when your muscles are cold i.e. in the morning when you first wake up. If your muscles are already warmed up and loose they will take the tension of the stretch away from those deeper connective tissues. It might seem counterintuitive at first because your body is so stiff in the mornings but that is actually the best time to have the greatest impact on the stretch moving into the deeper layers of your body.

That's not to say that practicing yin yoga at night is a bad thing either. It just has different benefits ;-)

Have all props ready, light a candle, play your favourite music.