The adjective “ater” goes just in the direction of “dark” in the sense of "mournful" / “gloomy”, since the “dies atri” were considered unlucky days (See Macrobius, Saturnalia, book 1, chapter 16, section 21, and Livy, Ab Urbe Condita, book 22, chapter 10, section 6 ), while “niger” means “black” and sometimes “malicious”/”bad”/” wicked “ (See Cicero, Pro A.Caecina, 27) or “inauspicious” as in “nigra avis” (bird of ill omen) and “funereal” as in “nigra hora” or “niger ille dies” with reference to death (See e.g.Tibullus and Propertius passim) .
- Livy, Ab Urbe Condita, book 22. chapter 10. section 6 : “si atro die faxit insciens, probe factum esto”
- Cicero, Pro A.Caecina, 27 : “Clodius, cui nomen est Phormio nec minus niger nec minus confidens quam ille Terentianus Phormio”
- Macrobius, Saturnalia, book 1, chapter 16. section 21:” Dies autem postriduanos ad omnia maiores nostri cavendos putarunt, quos etiam atros velut infausta appellatione damnarunt: eosdem tamen nonnulli communes velut ad emendationem nominis vocitaverunt”.
Note that Macrobius says that “dies atri”, aka “dies postriduani”, i.e. the days immediately following the Kalendae, Nonae and Idus of each month, are days of bad omens .
We know, in fact, that "Dies atri" were "dark" days in which fire should not be lit and sacrifices should not be offered in altars; temples should not celebrate public worship; all religious ceremonies are private but without sacrifices; starting new projects should be avoided, and certain gods, including Iuppiter and Ianus, may not be named.