Quote from manual:
"USCIS has 120 days from the date of the initial naturalization interview to issue a decision. If the decision is not issued within 120 days of the interview, an applicant may request judicial review of his or her application in district court. The officer must base his or her decision on the laws, regulations, precedent decisions, and governing policies."
Link: https://www.uscis.gov/policy-manual/volume-12-part-b-chapter-4
I'm sure they're just calendar days, but I wouldn't be surprised if USCIS interprets it as work days when required