In To Kill a Mockingbird, when does Judge Taylor appoint Atticus to defend Tom Robinson?

Although
it is obvious that Judge Taylor must
have appointed Atticus to be the defense attorney for Tom
Robinson long
before the beginning of the trial in 's novel ,
neithernor
 discovers this fact until just before the trial. Atticus
previously warned his children that
they may face trouble at school because
of the case, but he never tells them that he was
appointed as defense
attorney. They first learn of his appointment inwhen they sneak into the

courthouse hallway to see the excitement first-hand:

...The conversation was about my father. />    "... thinks he knows what
he's doing," one said.
   
"Oh-h now, I wouldn't say that," said
another. 's a deep reader, a mighty
deep reader."
    "... Lemme tell you
somethin' now, Billy," a
third said, "you know the court appointed him to defend this
nigger." />    "Yeah, but Atticus aims to defend him. That's what I don't like

about it."
    This was news, news that put a different light on things.
Atticus
had to, whether he wanted to or not. I thought it odd that he hadn't
said anything about it--we
could have used it many times defending him and
ourselves.

Atticus had previously told
his brother Jack in , but Scout apparently did not fully
understand their
conversation:

    "Atticus, how bad
is
it going to be? You haven't had too much chance to discuss it."
    "...
I
really can't tell at this stage, Jack. You know, I'd hoped to get through
life without a case of
this kind, but John Taylor pointed at me and said,
'You're it.' "

In the film version of
the movie, this fact becomes evident much
earlier, when Judge Taylor comes to
Atticus's house to ask him this favor
personally.