26 U.S.C. § 7524
Annual notice of tax delinquency
Not less often than annually, the Secretary shall send a written notice to each taxpayer who has a tax delinquent account of the amount of the tax delinquency as of the date of the notice.
Not less often than annually, the Secretary shall send a written notice to each taxpayer who has a tax delinquent account of the amount of the tax delinquency as of the date of the notice.