It is 45 calendar days; please see:
actual statute here:
(3)Requirement that property be identified and that exchange be completed not more than 180 days after transfer of exchanged propertyFor purposes of this subsection, any property received by the taxpayer shall be treated as property which is not like-kind property if—
such property is not identified as property to be received in the exchange on or before the day which is 45 days after the date on which the taxpayer transfers the property relinquished in the exchange, or
(B)such property is received after the earlier of—(i)
the day which is 180 days after the date on which the taxpayer transfers the property relinquished in the exchange, or
the due date (determined with regard to extension) for the transferor’s return of the tax imposed by this chapter for the taxable year in which the transfer of the relinquished property occurs.
Whenever a statute does not state "business days" or "court days" then it is presumed to be regular calendar days.