In many cases, the most effective time to fertilize is when you don’t expect any rain for two to three days. At this time, you should fertilize, lightly water your lawn, and let the fertilizer sink into the soil. If you must fertilize around rain, you should fertilize before a light rain, but after heavy rain. Light rain can perform the same function as watering your lawn after you fertilize, helping the fertilizer penetrate the soil. Heavy rain, however, can wash away fertilizer.