Minecraft 1.14 Raids

According to the Minecraft wiki, raids only happen if the player enters a village with the Bad Omen status effect. A raid is a special event that gets its own "boss health" bar at the top of the screen for each wave. You'll never encounter a raid if you never the Bad Omen status effect.

Sometimes, you'll spot Illager patrols roaming near a village, and if they get close enough, they will attempt to attack and kill your villagers. This does not count as a raid, though. That being said, if you kill the captain of the patrol (the one with the flag) and then enter the village they were attacking, you'll start a raid (since killing the captain will give you the Bad Omen).