There is no single best answer to this question, but there are a few factors to keep in mind:
If you plan on attending an Evening Sky Viewing, consider the phase of the Moon. With the Moon at any phase between several days before First Quarter and 3 or 4 days past Full, bright moonlight limits our ability to see fainter stars and the Milky Way. You can see a calendar of Moon phases at StarDate Online to help you make your plans. For exact phase dates & times, go to the U.S. Naval Observatory's moonphase page.
Though we can't predict the weather if you're visiting us weeks or months in the future, you can get some idea of what to expect by checking out our links to some of our favorite weather information sites.