The Fed has procedures that you should follow when disputing a late return allegation. In essence, you provide and document the cash letter information for presentment of the check and do the same for the return item letter. A lot of late return claims are wishful thinking on the part of the depository bank.
Just be sure that the holiday you refer to is one provided for in Regulation CC, and not some locally observed day that doesn't count, if it makes any difference to your case.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8