In astrology there is a rule called sect. This is simply a rule for the differences between day charts and night charts. To know whether you have a day chart you have to check whether your sun is above or below the horizon, that being the ascendant. So if your sun is in the (12th, 11th, 10th, 9th, 8th, 7th) house then it’s a day chart, and if it’s in any of the other houses (1st, 2nd, 3rd, 4th, 5th, 6th) then it’s a night chart. It’s useful to know whether you have a day or night chart not only for stuff such as calculating the part of fortune, but also for understanding the planets in your chart better. 

To start, the planets are divided into two groups or “sects.” Well, really only the traditionally used planets. The day sect includes the Sun, Jupiter, and Saturn, and is lead by the Sun. The night sect includes the Moon, Venus, and Mars, and is lead by the Moon. As you can see Mercury is in neither of those groups, for Mercury is traditionally seen as a very neutral planet. But it is said that if Mercury rises before the sun then it is a day planet, and if it rises after the sun it is a night planet.

When the Sun is above the horizon the day planets have more of an ability to influence and function in a productive way, while if the Sun is below the horizon the day planets have more difficulty with this. Same goes for night planets, if the Sun is below the horizon night planets have an easier time influencing in a productive manner, and if it is above the horizon they have a more difficult time doing so.

Sect also influences the qualities of planets. For example Saturn work more favorable (although still difficult) in a day chart, and less constructively in a night chart. Opposite applies to Mars, the other malefic, it works more constructively in a night chart and is more detrimental in a day chart. To use an example “Saturn in the 8th house, if by day, allots an increased income over a period of time […] if he is in this house by night the inheritance will be lost” (Firmicus Maternus.) 

Benefic planets are also effective, not as in that they become negative but that they become more restrained in their positive influence. Thus Jupiter is more beneficial in a day chart, and more inhibited in a night chart. Opposite applies for Venus, it is more beneficial in a night chart, and more inhibited in a day chart.

There are a few other little details, such as day charts being more influences by their sun sign and night charts more by their moon sign (although both are still really important in either chart). 

Also just to note although it isn’t that important compared to what else was stated here, night planets prefer being in yin signs (water and earth) and day planets prefer being in yang signs (fire and air). Also, day planets prefer being above the horizon during the day and below it at night, while night planets prefer being below the horizon during the day and above it at night.