Sun Zenith angle and location on earth
The Sun will be at your zenith a few days after 21st March, after it crosses the equator heading northwards* going towards summer, and a few days before 21st September, before it crosses the equator heading southwards* after summer has ended. 

*  from an Earth-centric viewpoint. We know, of course, that the Earth is rotating about its axis and revolving about the Sun, and that Earth's axial tilt is what causes the seasons and the apparent motion of the Sun across the sky both through the day and through the seasons.