How to label events chronologically from most recent?

Solution 1:

A formal term used in such situations is reverse chronological order.

money-zine.com:

Reverse Chronological Order Definition
The term reverse chronological order refers to a format that is commonly used in combination resumes and chronological resumes to display work experience or work history. The reverse chronological order format calls for the most recent work experience to appear first in the document, while the oldest experience appears last.

You can set the context (perhaps an always visible heading) as "In Reverse Chronological Order" and number the history items starting at 1.

If explicit labeling of each item is desired, they can still be numbered (and annotated) like:

First (in reverse chronological order) aka most recent
Second (in reverse chronological order)
Third (in reverse chronological order)
...