Every woman ovulates at different times of her cycle. You should not have sex on days that you skipped a pill and if you miss even just one pill you need to use a back-up method of birth control for at least seven days.
Also, sperm can live inside your body for five to seven days. So even if you are not ovulating on the day that you have intercourse does not mean that you won't be before that sperm dies.
Be very careful. Even taken correctly birth control is only 99% effective. If you miss pills you need to use a backup method.