Day means a day from Monday through Friday, including State and local holidays but not Federal holidays, for any time period less than 11 days specified in this part. For any time period greater than 11 days, day means calendar day.
49 CFR § 1552.1
Scope. This subpart applies to flight schools that provide instruction under 49 U.S.C. Subtitle VII, Part A, in the operation of aircraft or aircraft simulators, and individuals who apply to obtain such instruction or who receive such instruction.