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Directs focused on specific topics don’t have guidelines as to optimal air time.
Normal Directs happen on Thursdays because that’s the day the eShop updates. Pokémon Directs don’t need to follow that. For example the Direct that announced X and Y was on a Tuesday. So was the one that announced USUM among other things. The one that aired for the 20th anniversary aired exactly at midnight on the 27th of February in Japan, which was a Saturday, and in other regions it was midday on the 26th, a Friday. That’s very likely what will happen this year as well.