I would translate it as something like:
To anyone whose turn may come, may they shit during the day and not at night. -or-
To all those who get a turn, I hope they shit during the day and not at night.
Al que le toque (subjunctive used since the person "al que" refers to is indefinite; it's like saying "to who(m)ever" instead of just saying "to who(m)" since the latter is a definite person).
...que cague (subjunctive because it expresses the will of whoever is saying it).
Well, I hope this helps, but be sure to write again if you still have doubts.
Thanks! What was confusing me most of all was the second que. I couldn't tell what its grammatical function was, given the context. And the translations provided make sense within the context of the story.