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If there is an ideal time to use neem oil on plants then it would be late at night or early in the morning.
Very high summer temperatures in the late morning and afternoons can have a cooking/baking type effect on your neem oil covered foliage. As well as picking the best time, try locating your plants away from direct sunlight during application.
Another reason to pick the right time would be because plants get visited by bees and other beneficial insects during the daytime.
All that said, consider too the pest you are targeting when thinking about time of application. One non-UK pest springs to mind, the ‘Colorado Potato Beetle” If your neem spray dries out in the sun then the beetle wont be affected. The Colorado Potato Beetles responds best to damp/wet spray.