The game takes advantage of day and night cycles.
Zombie behavior is not the same around the clock.You'll find zombies during the day slower, more docile and lurking in the shadows. Secure a structure or roam free, the choice is yours.
The horde system keeps track of a few statistics including how many 'horde nights' you've survived, how many zombies you've killed when the horde starts and stops along with the direction the horde is approaching from.