If you're trying to conceive, you need to time intercourse around ovulation to have the best chance of pregnancy. It can be tricky because your ovulation date can change cycle to cycle even if your cycles are regular. A lot of women will have intercourse daily or every other day after their periods leave to increase chances of conception.
I charted several cycles using basal body temperature charts (the book "Taking Charge of your Fertility" explains this) as well as having intercourse whenever I noticed my body producing fertile signs of impending ovulation, such as vaginal discharge that is very stretchy and resembling uncooked eggwhites and ovulation pain. You can also use ovulation predictor kits (OPKs, sold in the drugstore) to predict when ovulation is approaching.
The cycle I got pregnant, I only had intercourse one day during my fertile period, which happened to be the day before ovulation, and that did the trick

Sperm can survive for up to 5 days before ovulation if conditions are right, but for most women, the most fertile days of the cycle tend to be the 1-3 days before ovulation and the day of ovulation itself. The day before ovulation is supposed to be highly fertile.