With a 28 days circle which days are my most fertile days my last period started on 25th of last month. What is the chance of pregnancy when one has sex a day after ovulution?

There is no way to know for sure when you are most fertile. You can use this site to try and check when your fertile times MIGHT be (Ovulation Calendar | March of Dimes (http://www.marchofdimes.com/pregnancy/ovulation-calendar.aspx?gclid=CNy0_Iam5LoCFXHxOgodfkgA-A)) but it will only be a guesstimate.

There is also no way to calculate the odds or chance of when you might get pregnant.

If you are having trouble conceiving you should discuss it with your OB/Gyn or a fertility specialist.