Nomads, Merchants, and Necromancers spawn in the four corners of your village, about 120 blocks from the town hall.
Each day the Merchant will choose a corner. If he cannot spawn in the corner, you will receive a message in chat saying "Merchant has failed to find a way to village."
Nomads will do the same once they have determined if they spawn. They may choose a different corner to spawn than the Merchant.
If the Necromancer is able to spawn, he follows the same pattern. You will receive a chat message that he has spawned.