Most people who are even remotely interested in astrology know that if you want to begin something, then it's best to do so on a new Moon, this also includes exorcise programs and kick-starts to any health regime we may be wanting to engage in. The reason for this is if we start with the growing light of the Moon chances are our efforts will have more of a chance to grow.
In general, if you want something to become bigger, more prominent and more of a ritual in your life, the best time to start is when the first light of the Moon begins. This is thought to be the time when the idea or intention has been made conscious, it’s emerged into the light of our awareness and can be made manifest in the world.
In the Ashtanga lineage of yoga, the only two days a month that you don't practice are on the new and full Moon, as both are too extreme. The new Moon correlates with the downward pull of energy -- apana -- at the end of an exhale, while the Full Moon correlates to the upward moving energy -- prana -- at the peak of an inhale. These are good days to rest or do a few restorative poses at the most.
If you are starting a yoga practice for the first time or reigniting an old romance you once had with it, start slow at first. Allow your first few days back on your mat to be in either simple beginners classes or even at home to a DVD or your own sequencing.
As the Moon begins to grow in light, consider upping the anti and experiment with different styles of classes, different teachers and different studios and doing your practice at different time of the day until you find the right fit. It's important that you feel comfortable in the classes that you choose, otherwise it becomes consistently harder to return and steadily easier to find excuses not to.