In Shakespeare's Macbeth, when does Macbeth kill Duncan?

In Shakespeare's Macbeth, as is typical in Shakespeare, violence takes place offstage.  The audience does not see Macbeth kill Duncan any more than we see Macduff kill Macbeth later in the play.  At the close of Act II, Scene 1 Macbeth says:

I go, and it is done:  the bell invites me.

Hear it not, Duncan, for it is a knell

That summons thee to heaven, or to hell.

Then he exits.  He returns to stage in scene 2 after only a brief absence and tells Lady Macbeth, "I have done the deed."  The results of the assassination are revealed to the audience for the remainder of the play, but the act itself is not seen.

Just so you know, I edited the question in order to reflect the accepted convention of referring to details within a work of art in present tense rather than past tense.  Thus, "does" works better in the question than "did."  When you analyze literature, think of it as if it's happening whenever it's being read, or in this case, maybe, performed.