Since 1.2 i noticed that hardmode bosses can spawn every night much like the Eye of Cthulu in older updates. Its really frustrating so if you see these messages know what is coming:
"This is going to be a terrible night..." - The Twins will spawn.
"The air around you gets colder" - Skeletron Prime will spawn.
"You feel vibrations bellow you" - Plantera flowers will start spawning in your jungle.