Definitions, definitions, definitions!
The term "business day" is defined in Section 1026.2(a)(6) in a particular way for its use in the ROR rules in Section 1026.15(a)(3) and 1026.23(a)(3). The definition incorporates 5 U.S.C. 6103(a) which declares the exact days for the observance of the 10 federal holidays.
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...gone fishing.