Yes, you can predict your fertile days with 28 days cycle but can not say with authority. Ovulation usually takes place at mid of the cycle but some ladies can have late and some early ovulation. In a 28 days cycle, ovulation day can be 14th day starting from first of periods. As sperms can live alive for 5-7 days inside female body after ejaculation during sex, an intercourse 3-4 days before ovulation can also lead to pregnancy. For a 28 days cycle period, fertile days can be 11, 12, 13, 14 and 15 of the cycle.