What time do you start saying "Dobry wieczór"?
My book said szósta godzina, but that seems early.

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A say "Dobry wieczór" when it's getting dark:)

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I think that 18-19

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In Russian the phrase is relevant from 18:00 to 00:00, which is the same in either English or Polish, I think.

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There are no clear boundaries on at what time you should start saying "Dobry wieczór" ;) I guess you're right that six is a bit early, but perhaps seven, eight..