Day 7 is more or less the time that decides if the player's defenses are solid or not. On the night of every 7th day (7, 14, 21...), a massive horde of Zombies travels through the area the player is currently in.
There are also roaming hordes that spawn both during the day and at night, though they do not detect the player automatically by default. The roaming hordes start on day 1 with a single zombie and then three zombies at night. This slowlly ramps up through day 5. When the roaming hordes come back on day 9, they have grown a great deal. On some days, the roaming horde will be a small pack of Zombie Dogs . This continues through day 19 before resetting on day 22, which will have roaming hordes identical to day 1.
Blood Moon Hordes
On every 7th night, from 22:00 to 4:00, a large horde of zombies will spawn. Also known as a Blood Moon Horde, because this is distinguished by the full moon being a blood red color, the area outside will also ahve a light mist with reddish tint. This horde of zombies will be able to detect the player through walls and when underground, even when no noise is being made, the Heatmap is low, and no odorous foods are being carried. On the seventh night, this horde will contain only normal zombies with a small number of Zombie Dogs . The horde on night 14 will be larger. On the 21st night, Zombie Bears , Burn Victims , Infected Police Officers , and Feral Zombies will join the horde, which will continue to grow in size every week through night 48.
If a Screamer Zombie sees a player they can call up a horde. These hordes can be similar in structure to a Blood Moon horde, with Zombie Dogs , Infected Police Officers and all the other types, based on the player character level. Screamers are attracted by the Heat Map of the area, so high levels of activity will cause them to appear. Even a summoned Screamer can scream to attract another horde of zombies of various types.
Post Blood Moon Horde Hordes
After a blod moon night there's a chance for a Post Horde Horde to appear, at least in single player. It may consist of such animals: Zombie dog, Zombie Vulture, Wolf and possibly other animals.
They follow hordes early in the morniing usually, before 12 A. So far there have been observed 4-6 dog hordes, 7 wolves hordes (including 2 grey wolves), and 12 vulture horde; the last one is the least dangerous. They are not necessarily programmed to cross player's path, as vultures were very passive, but this could be due to programming limitations of vultures.
Combat and Defenses
With the roaming horde, it is quite tricky but not impossible to dodge them completely. Zombie Dogs are extremely fast, even in daytime, and can catch you quickly if caught offguard. It is not reccomended to hide in a house without heavy defences and very durable walls if the player has spotted, because of the infected Police Officer's corroding vomit and large amounts of zombies hitting the walls. This could possibly bring the house, and the player, to the ground.
A Shotgun with plenty of Shotgun Shells and good dodging skills will allow the player to take down most of the zombies. A .44 Magnum with plenty of ammunition will make quick work of the horde. Pipe Bombs are also a big help because the zombies tend to group up pretty closely. It is reccomended that all Zombie Dogs be killed first, because of their speed. After that, Infected Police Officers will likely be in range and can be killed and possibly looted. Feral Zombies should normally be avoided and saved to be killed last because they drop the best loot and are quite the ammo dump.