There's a graphic on page;
http://www.nacha.org/About/what_is_ach_.htm that shows the process. And here are the 'rules'
http://ebt.nacha.org/docs/Chapter_6.pdf
"Example 2: Transactions performed after
Thursday Business Day cut-off time and before Sunday
cut-off time shall be settled to Acquirers on Monday
Business Day)."
Hope that helped.