Never spray on a windy day. Even moderate wind will prevent you from accurately spraying your target plants. And pesticide and herbicide can travel a considerable distance once picked up by the wind. They may end up on non-target plants or worse, in your neighbor's pool or yard. There is no way to predict wind speed on a given day with 100% accuracy. However, by following weather patterns and using a bit of commons sense, its easy to pick a day that will be most suited to spraying your plants.
Investigate the wind forecast for your area. Most local news broadcasts and newspapers list the daily and weekly wind forecasts. If your local news does not offer this, or you do not know where to find it, there are many online options. Websites like Weather Underground allow you to search for wind conditions and forecasts in your area by zip code.
Predict on your own. Since you will likely spray your plants at roughly the same time every year, you can make your own personal wind chart to track wind patterns in your area. Use the sources in step 1 to track wind patterns two weeks ahead and two weeks behind your traditional spray date. In the future, you will be able to look back at this log to predict when the best day for spraying may be.
Spray early in the morning. You can safely predict that wind speeds on any given day will be at their lowest in the early morning. The sun has not yet had a chance to heat the atmosphere and cause convection currents. These convection currents are responsible for those unexpected gusts of wind that blow your spray around.