Not later than 5 days after the day on which an assessment is made under 26 U.S.C. 6862 or when a levy is made less than 30 days after the notice and demand described in 26 U.S.C 6331(a), the officer who authorized the assessment or levy shall provide the taxpayer a written statement setting forth the information upon which that official relies in authorizing such assessment or levy.

(26 U.S.C. 7429(a)(1))