Cash basis is misleading, especially for this case. GAAP accounting would have the revenue deferred until recognized when the event occurred. It's usually not a big deal unless payments and recognition cross reporting periods, like it did here.

This will likely mean 2021 convention financials will show a big loss because we reported the revenues last year and will have the expenses this year. You'll need to add the revenues and expenses together for 2020 and 2021 to find out how the convention did overall.