The short answer
You are most fertile in the two to three days before ovulation and on ovulation day. Sperm can survive up to about 3 days and an egg about 1 day after release. With regular cycles, ovulation is about 14 days before your next period, so sex every 1–2 days in that window gives the best chance.
Natural-cycle calculation is not reliable as contraception. See ovulation tracking.
Answered by Dami Women's Clinic (다미산부인과의원), Yeouido, Seoul · Dr. Ko Sang-hyun, OB/GYN. General information, not a diagnosis. Emergencies in Korea: call 119. Medical disclaimer · International Patient Center