This page of the Dying Light 2 game guide contains information about the day and night cycle. You will learn how it affects the game and how to change the time of day.
While wandering the streets during the day, you will come across bandits and hordes of zombies. They aren't particularly aggressive. You can easily deal with single enemies. You'll have a harder time facing entire groups. During the day, however, you should have no problem escaping, because zombies don't have as much strength.
Your opponents will often be hiding in locked buildings. In these locations, you can find important items, such as epic weapons.
More difficult opponents appear on the streets at night. They can run fast, climb onto rooftops, and even call over their other companions. Fighting these enemies isn't recommended, instead try to run away from them.
It's worth noting that traversing the city at night gives you additional XP and allows you to reach locations that may be unavailable during the day. This applies, for example, to buildings that become empty after dark when their zombie occupants leave to go hunting.