The meaning of immediately before used in section 283 of the Income Tax Act is not defined in the act itself. But we can take guidance from decisions by various high courts. Here is an excerpt of the court’s order in which the Hon’ble High Court tried to explain the meaning of immediately before.
Meaning of immediately before defined by court
The phrase ‘immediately before‘ used in section 283 of the Income Tax Act came up before hon’ble Madras High Court in Thangam Textiles Vs. First ITO  90 ITR 412 (MAD.) and it explained.
Meaning: The phrase ‘immediately before’ means ‘preceding the date of dissolution’ and has no reference to the year of assessment.
Section 283 of Income Tax Act
Service of notice when family is disrupted or firm, etc., is dissolved.
283.(1) After a finding of total partition has been recorded by the Assessing Officer under section 171 in respect of any Hindu family, notices under this Act in respect of the income of the Hindu family shall be served on the person who was the last manager of the Hindu family, or, if such person is dead, then on all adults who were members of the Hindu family immediately before the partition.
(2) Where a firm or other association of persons is dissolved, notices under this Act in respect of the income of the firm or association may be served on any person who was a partner (not being a minor) or member of the association, as the case may be, immediately before its dissolution.
In this article, you can get guidance from high courts on the meaning of immediately before under Section 283 of the Income Tax Act.
Updated up to Finance Act 2021.