From the Pentagon website:
To take a guided tour of the Pentagon, you must make a reservation in advance. Note that tour schedules can fill up quickly, so it is advisable to book your tour well in advance of your visit. Reservations may be booked from 14 to 90 days in advance. Reservations will not be accepted for tour dates within 13 days or more than 90 days away.
Hours: All guided tours of the Pentagon are free and available by reservation only. Tours are conducted Monday through Friday from 9 a.m. to 3 p.m. Tours are not conducted on weekends or federal holidays.
Book a Tour: Tours can be arranged directly through the Pentagon Tour Office using an online reservations system.
Group Size Section: Public Tour Groups will NOT exceed 60 persons.
Request a Pentagon Tour