I have a 28 day cycle

The average observed times are two weeks after the start of your period. At this point it would be the 30-31st depending. The problem is that it is an average and not absolute for EVERY woman. Some ovulate soon, later, or not at all.

Google "basal temperature ovulation" It will show how you can roughly tract your ovulation cycles. Also pay attention to the warnings and limitations of doing this.

It is best to try to have sex every 2-3 days. You'll hit ovulation, but also realize that it can take quite some time to become pregnant too. Often up to a year or more.

I agree with CravenMorHead on this... while the question may appear simple... there is no simple answer. For the stated reasons.