When we liked some picture sometimes we don't know why - maybe its good composition, colours, connection between subjects...And LIGHT!
Light made picture special and magical. Its a first thing what you need consider when you ready made a click!
Best time for pictures
Thats why most photographers choose to schedule their sessions in the best light conditions, often it is 2 hours before sunset (or after sunrise) - Golden time for pictures. Or early morning when light so beautiful, lay down and give nice warm feeling!
Time during the day not so great because of bright sun what cause bright spot on the face and deep shadows. At this case we could take shots in open shade (under the trees, near the buildings) or at covered shade (under the porch, garage, umbrella etc).
Overcast weather good for pictures at any time, clouds work like diffusor, made soft light. Only one point - sometimes its could be hard to get nice catchlights, so need position your subject right way, even look up to the sky - and you get great catchlights!
If you want take a pictures inside some space (house, office, church etc) first you need look at available light source (even small window can give you desperate light). If you have choice and a lot windows available you can position your subject at the window with nice soft light (check sun position - its might be from opposite side of the house). You can play with light and shade on the face just with position your subject at the corner of the window (more light), in the middle (deeper shade on half face) - turn face at 90, 45 degrees forward the window or from it. Also you can use a reflector (its could be any wight, silver, gold surface), also you can play with black side of reflector (what take light as a flag and cause more shade).