There is no such thing as first (or 1st) annual.
The event can be inaugural in the first year, but never annual.
“An event cannot be described as annual until it has been held in at least two successive years,” according to the AP Stylebook. “Do not use the term first annual. Instead, note that the sponsors plan to hold an event annually.”
As a plea to all public relations professionals, marketers and those organizing a local charity 5k, please stop using first annual. Be proud of the first event!