Its is a sorcery so you can only play it during your own main phase when there is nothing else on the stack. (unless you have something like Vedalken Orrery
that lets you cast sorcerys like instants)
For it to be any actaul use (ie you can attack with the creature) you need to cast it in your first main phase. This has a default stop as it is when you also normally play your lands etc.