the highest probability of fertilization is between 9th and 20th day from after menstruation, supposing the cycle has 28 days.
if it's longer/shorter, the range is longer/shorter as well. if the cycle is irregular, you can't count on that at all.

the ovulation (release of an egg) usually occurs between 12nd and 15th day and it can be theoretically fertilized from this time till the end of the cycle. however, the sperms can survive a few days inside and wait for an egg to be released, and the 'older' egg is 'more difficult' to be fertilized -> therefore the range is like 9--20th day.