15, the magic number. A new study suggests that the day in which is more likely to have a successful intercourse (in terms of reproduction :D) is the 15th day after starting the period. However we would consider too risky to rely on a single intercourse to get pregnant. This is why we usually recommend to try three times separated by 24 hours to allow the sperm to recover from each ejaculate. This would mean that the best way to naturally conceive in a patient with 28 days cycles is to have sex on days 13, 15 and 17 of each cycle.
http://humrep.oxfordjournals.org/content/28/4/1110.short?rss=1