Díky. But in English, "a sad day" is not literally referring to emotion (a day of mourning) but to unfortunate events. E.g., "It was a sad day for the US when Donald Trump was elected president." That statement is separate from a statement about whether people were sad or happy; rather a commentary on a misfortune. So my sense is that "sad" is used here in the same sense as "nešťastný" in the Czech phrase.