Here's a good thread on this (below). My opinion is that if you don't credit on Saturdays (reason given above), that providing a disclosure describing your bank's practices complies with 12 CFR 226.36 (c).
Your case is exactly like ours. We close at 4PM. Here's our statement for principal dwelling secured loans: "Credits to your Account for Payments on your loan are subject to certain timing rules. If payments are received by the bank by 4:00 PM Monday-Friday, they will be credited to your account that same day. If received after 4:00 PM Monday–Thursday, they will be credited on the following business day. Payments received after 4:00 PM on Friday, or on Saturday will be posted within 5 business days, but in most cases on the following business day."
Good luck!
http://www.bankersonline.com/forum/ubbthreads.php?ubb=showflat&Number=1370308