Day one of the cycle is the first day bleeding from a period starts.  TTC
should take place ideally up to 5 days before ovulation up to the day of
ovulation.  Since ovulation takes place at the midpoint of the cycle, which is
day 14 of a 28 day cycle for the prototypical average woman,  the best days for
TTC is days 9-14 or a few days after bleeding stops.  If your cycle is shorter
or longer than 28 days, then add or subtract that from the  9-14 days cited
above for the best days for TTC.

This is all just based on cycle length, there are a variety of other indicators
you can use.  An excellent book on the topic is "Taking Charge of Your
Fertility".

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