The obvious answer is, "Whenever you can." But I really like to know as much about the weather as possible when planning. Weather Underground (http://www.wunderground.com) has recently upgraded their website to include a 10 day forecast with temp graphed, chance of rain, snow, etc., and you can customize for barometric pressure, cloud cover and humidity. I have a reporting station with 2 or 3 miles of my house. This is a screen shot of a few minutes ago for my location.