What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
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Баллистические ракеты, разработанные в Иране, способны долететь до целей на территории Европы. О возможностях иранского оружия доктор военных наук, капитан первого ранга запаса Константин Сивков рассказал изданию News.ru.
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Again, the answer to this question varies from company to company. And again, testing is the way to find out what works best. Typically, weekends and mornings seem to be times when multiple emails are opened, but since your audience may have different habits, it's best to experiment and then use your data to decide.