I still haven't found where the BBC, or any other news agency, have reported that "Jeremy Corbyn denies some of the findings" but I did find the following while I was looking

It (the EHRC) found Labour responsible for three breaches of the Equality Act:

Political interference in anti-Semitism complaints

Failure to provide adequate training to those handling anti-Semitism complaints

Harassment, including the use of anti-Semitic tropes and suggesting that complaints of anti-Semitism were fake or smears

So, to me, that suggests that the "Day of Shame" refers to how complaints were handled and harassment of those complaining rather than any institutional racism. Once the report has been fully digested I would be interested to see if it suggests that antisemitism in the Labour party is worse than in any other large organisation.