Blood moon fires on day seven and every seven days after. The horde is larger each time, and it is not wandering. It is pathing toward you.
The cycle
You get six days of quiet, then one night of pressure. Server settings can change the interval, but the rhythm stays.
How the horde targets you
Zombies path toward your position and attack whatever blocks them. That is why a decoy wall works and why an unsealed doorway does not.
Scaling
Horde size grows with your game stage, which rises with level, days survived and biome difficulty. Getting stronger also makes hordes harder.