unsafe day; fertile day
Refers to a day in the menstrual cycle when unprotected sex is likely to result in pregnancy. Often used in the context of contraception or family planning.
Today is an unsafe day, so be careful.
Avoid intercourse on unsafe days.
排卵日 specifically means ovulation day, which is the most fertile day. 危険日 is a broader term covering the fertile window around ovulation.
Compound of 危険 (danger) and 日 (day). The term is a straightforward modern coinage for family planning contexts.