Where is the location of this the latest 'Holding Day' definition so that I may read more?
The extent of any historic calls is limited by the Consumer Data Right Legislation. It is referred to as the holding day and is the 1st of January not more than 2 years earlier than the Designation instrument (as per s56AC(4) of the Competition and Consumer Act 2010).
The earliest holding day for the CDR in banking is 1 January 2017. This is specified in paragraph 5(3) of the Consumer Data Right (Authorised Deposit-Taking Institutions) Designation 2019.
See also Historic Records 'Oldest-time'.