Generally speaking, bright sunny days are best for disposable cameras. That will be when the colors and contrast shine the most. It is also when the fixed exposure settings of the camera make the most sense. However, the cameras with higher film speeds will open you up for more lighting conditions like cloudy days or some shadowy areas.  If your camera has a flash, you will be able to shoot in more lighting scenarios. In many cases, it’s actually easier to shoot with a disposable in very dark settings because the camera doesn’t need to autofocus, which requires enough illumination to create contrast.