SGC, if your cycle is around 34 days or so, you are likely ovulating around day 20.
Cycle lengths show significant variation from woman to woman, but a more consistent measure is the ‘luteal phase’ of the cycle, which is the time after ovulation until you get your period. This is almost always 14 days (unless you have a luteal cycle issue) so chances are that you are ovulating close to day 20 which means that days 18,19,20 are most likely your most fertile.