The user would have the option to set up a new "source" of revenue themselves to track all sales made at a specific event. When creating an order for a guest/registered customer via the app or via the POS, the user would be able to select this event and record the sale as usual.
As a winery doesn't always close the cellar when participating to a wine event, the user would then have the option to use both options (at the same time) when recording sales.
This could be a matter of login in the POS with specific login details, or have an option to click the event name in a drop down menu when processing the sale.
As a result, the End of Report period would show the total of sales made during this specific event.