unlucky day; bad day
Refers to a day when things go wrong or misfortune is expected, often in a superstitious or colloquial sense.
critical day (for crops)
Agricultural term for a day when weather or other conditions are especially critical for crop growth or harvest.
Today is an unlucky day. Nothing has gone right since this morning.
Some people avoid making important decisions on unlucky days.
For farmers, a day with frost is a critical day for the crops.
Compound of 厄 (やく, 'misfortune') and 日 (ひ, 'day'), with rendaku voicing of ひ to び. The exact historical origin is uncertain, but the word has been used since at least the Edo period.