It is day 14 of the menstrual cycle, but day 1 is the first day of your period.
If you have your period every 28 days and your period starts on the 1st of the month, your most fertile time is the 15th.
If your period starts every 30 days and your period began on the 1st, the most fertile time is the 17th.
So roughly half way between the start of two periods.
There is a handy tool here which will do the calculations for you
http://health.yahoo.com/pregnancy-getpregnant/ interactive-tool-when-are-you-most-fertile/hea lthwise--tn9893.html