Scaredbaka here is a text book example of a womens cycle.

First day of period = day 1
ovulation = day 14
so the most fertile time would be from day 7 to day 21

a man makes sperm on demand but if demand is greater than supply you get diminished sperm production

there fore if you are trying to concieve it is recommended to have sex every other day (to let the number of sperm build back up) staring around day 7 onwards, meaning that even if you don't then have sex on your ovulation day there is still some sperm hanging out waiting for an innocent egg to just pass by

an egg will last for upto 48hours before passing out the womans body, sperm can live in fertile conditions for upto around 5 days (some say 7 days).

There, is therefore, the potential if you had sex on day 7 and ovulated on day 14 that there may be enough sperm left to fertilise the egg, and if you ovulated on day 14 and didn't have sex until day 16 it is possible the egg might just still be there.

Add into that the fact that ovulation can vary with each cycle and you can see why if you use charting or natural methods of "contraception" that you a) have to be very careful and committed and b) that you have to be able to accept the consequence of a little slip up.

All this can be avoided by using condoms (even better if they have a spermicidal lubricant or if you add a spermicidal lubicant yourself) and increasing that safety by also taking birth control pills, depo shot, patch or implants.
There are other methods of contraception but only barrier methods such as the condom or female condom also protect against infections as well.