To pick a good day for fishing, read fishing reports and weather forecasts for your area. Look for fishing reports that indicate fish are biting, and look for forecasts of cloudy weather or light rain. If you want to catch a certain type of fish, research the weather conditions that make the fish most active. Choose weekdays when possible, as they're usually less popular.
Light causes fish to hide and reduce their activity levels, so it's more difficult to catch fish when the sun is out. Clouds reduce the amount of sunlight that reaches fish, so they move around more on cloudy days. A light rain obstructs the view fish have of the surface, making it harder for them to see you through the water. Rain also transfers insects to the water, making fish more active and more likely to bite on your bait.
When fishing in saltwater, check the tide schedule in advance. Rising and falling tides tend to have the most fish. Movement and noise scare fish, so it is more difficult to catch them on populated bodies of water. Fishing is most common on weekends, making weekdays a better choice. Heavy rain and storms are poor conditions for fishing. Heavy rains can make water dirtier and clog fish gills. Storms, especially those with lightning, create unsafe fishing conditions.