It shall be the duty of the commissioners, acting as a board of revision, or the board for the assessment and revision of taxes, as the case may be, to hear appeals at any subsequent time when they may be in session, previous to the payment of the tax, and to make such alterations as they might have done on the regular day of appeal: Provided, That no such appeal shall be heard unless the appellant shall have given due notice thereof to the assessor of the proper ward, borough, town, township or district.
72 P.S. § 5020-517