Since all beginner photographers start out shooting Ambient Light (natural light) it is best to put your self in the BEST light so to speak for great portrait work. So the time of day matters most and the light at the begging and end of the day is best. Also days that have consistent thin clouds make for some great soft light. So to recap...

Shoot at the beginning or end of the day

Choose days with consistent thin clouds for amazing soft light

Also try to create images that shape the face. Flat light or even light across the face doesn't shape the face and won't make for a more interesting photo.

Happy Shooting...