Is there a field that lets you need when a Credit Memo was applied? Not looking for the transaction date, or the created date, but when it was actually applied against an Invoice for example. I can't find much with googling.

You are looking for the date the credit memo was applied to the Sales Order?

Applied to the invoice.

If you open the credit memo and look in the system information, is that date you are looking for there?

Yeah I was looking at that and it doesn't seem like anything triggers in the system notes when applying or un-applying an invoice against he credit memo.

If you apply enough of the CM to fully close the invoice, system notes captures the status change on the invoice. But if it only partially applies, you might be out of luck. The "Amount Remaining" field would change on that day too, but I'm not sure that's captures by System Notes.

there is a “hidden” table(s) Related Transaction Lines (Next and Previous) In a nutshell, it is the same table from two perspectives