Seasonal: It is an environment thing. In the winter, it is traditionally dry and cold. Therefore, a light, citrus would not do well in the environment. A darker fragrance is required to hold a body in the environment. Vice versa, if you use a deep fragrance in the sunny, humid environment it would be overwhelming and nauseating.
Day: A light, but sometimes dark, fragrance that can be easily transitioned from work to errand would be a daytime. A night fragrance would be a darker, deeper (sometimes sultry if clubbing) to fit the setting and mood.
Now keep in mind that all of these are personal preference, but trends are what dominate the fragrance world. So if you smell the new fragrances during the summer you will smell a lot of citrus and clean. Fraiches and aquatics. But as soon as fall hits fragrance launches change to heavier fragrances.