Ideally, if you can have sex during the days prior to ovulation and ovulation day, you will give yourself the best chances. Sperm can live for a few days in you body once they can get past the vagina and cervix which are hostile can tend to kill off the majority of them. For the swimmers that make it past that point, they can hang out in your fallopian tubes for a few days waiting on the egg. If you've had sex in the few days prior to ovulation, say 1-2 days, chances are there will already be sperm waiting for the egg. Having sex on your ovulation day helps to boost those odds, so you have swimmers waiting and sending additional ones on ovulation day.
I was told by my OBGYN on the 5 days prior to ovulation, have sex every other day and then on ovulation day. If there are no impeding factors/infertility issues, the average woman has a 20% chance of getting pregnant each month while having sex during her fertile window.
Good luck and I hope this helps.