When to Watch?

Deadbones1226 said:
I remember watching it after s2. Even though if I remember correctly, it takes place somewhere around late s1. It does indeed spoil a few characters from after s1, so you'd better leave it for at least after s2. And the disposition of a certain character is quite unclear if you watch after s2, but that makes no real impact on the flow/story. Also make sure to watch the s1 specials before it (quite boring and barely relevant but still if you're willing to make a watch order why not).

About its connection to the movie I think it was nothing important, like a character making a cameo at most.

Of course by S2 I mean season 2 part 2, not only the first part

If you want it completely spoiler-free tho, watch it somewhere mid s3 (after ep11 or so).