How likely is pregnancy to occur on day 20 of a 28 day cycle?

Pregnancy has been documented from exposure on any day of a cycle. It is more likely mid cycle, but it does occur earlier or later.

Pregnancy is known to have occurred from sex on any and all days of the menstrual cycle, including when the woman is bleeding. . If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex : https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines Get HPV vaccine