Fertility days

Hi,
Greetings of the day
My name is shalini.as I know that during ovulation if women become pregnant if she intercourse.I want to know how one should know this is the fertility day like my last period day was 9 mar 2014 my cycle is 28 days so when is the fertile day also I would like to share that every alternate day after my period we both intercourse only 4 days twice times in a month we could not did any thing so it might be possible that 4 days was my fertile day and I missed that day.pleas guide me .

Thanks
Shalini