There are many ambiguities in natural language, many of which are resolved via context.
In your example, you connect the word next with the word last by comparing the next event against the last. This makes the last a past event relative to the next event, and not the one at the completion of the series of events (also called the last event). That the last event is also the first event is irrelevant.
Are there any guidelines on which of the two should be used in this scenario?
The event to which both last event and first event refer is clear from the context, so ambiguity is not a problem. Since you're using that event as the basis for evaluating the next, the natural pairing is next vs previous or next vs last. If, however, the first event was particularly noteworthy (famous keynote speaker, fireworks, etc), then it would make sense to use the first event as the benchmark for comparison.