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 program, consider the phase of the Moon. A bright Moon changes what you can see in the night sky. 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 check 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 moon-phase page.
Other timing factors to consider:
the Summer sky has different objects visible than the Winter night sky
Sunset is later in Summer and evening programs take place 9:30-11:30 PM
Sunset is earlier in the Winter and evening programs take place 7:00-9:00 PM
We have a rainy season July-August and there is a higher chance of cloudy or rained out evening programs. You can check out links to some of our favorite weather information sites to help plan.
Check the program calendar for upcoming events and activities.