Pregnancy can't happen on just any day of the month. The fertile window exists because of two biological clocks: sperm can survive in the female reproductive tract for up to about five days, while the egg survives only 12–24 hours after ovulation. Put together, that gives roughly six fertile days per cycle — the five days leading up to ovulation, and ovulation day itself.
Using it
The highest-chance days are the two or three days immediately before and including ovulation — intercourse then means sperm are already waiting when the egg is released. Ovulation kits (detecting the LH surge), cervical mucus changes, and cycle tracking all help locate the window; regular intercourse every 2–3 days through the middle of the cycle covers it without turning conception into a scheduling exercise.