unlucky day; bad day
Refers to a day when things go wrong or misfortune is expected, often in a superstitious or colloquial sense.
今日は厄日だ。朝から何もかもうまくいかない。
Today is an unlucky day. Nothing has gone right since this morning.
critical day (for crops)
Agricultural term for a day when weather or other conditions are especially critical for crop growth or harvest.
農家にとって霜が降りる日は作物の厄日だ。
For farmers, a day with frost is a critical day for the crops.
厄日には大事な決断を避ける人もいる。
Some people avoid making important decisions on unlucky days.
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.