Has to be placed while the date is still ahead, which makes your own timing half of what actually gets tested.
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Most tests here can be run whenever it suits you. This one has to be placed while the event is still ahead of it, because responses that arrive after the date have nowhere useful to sit. That makes timing part of what is being tested, and it is worth treating your own planning as half the experiment.
Check the response figure on the event page from a browser where you are not signed in, before and after. Organisers see a different view with more detail in it, and comparing an organiser view against a visitor view is the most common way people confuse themselves here.
Nothing about who arrives on the day. A response is a signal shown on a page, and treating it as a headcount will lead to a bad evening. The honest use of a sample is to confirm that the figure responds to an order at all, and to see how it looks to somebody deciding whether the event is worth their time.
Short answers to what people ask most before ordering.