The easiest way to answer this question is when there is glowy light! What is glowy light? Glowy light is light which flatters. This type of light happens usually earlier in the morning ( just after sunrise) or during golden hour (hour to 2 hours before sunset).
So why do I as a photographer recommend this glowy light? Remember how I said it flatters. This light which doesn't cast awkward shadows on the face or other parts of the body. Midday light tends to be more direct/harsh and casts shadows in places like under the eyes, chin and sometimes other places. Glowy light isn't direct so it doesn't cast these shadows.
Next to glowy light, the next best light for portraits is overcast lighting. Overcast lighting where the sky is overcast due to cloud cover. The overcast sky acts like a diffuser for what might otherwise be harsh/direct light.
For more examples of an engagement sessions during glowy light, go to:
For more examples of weddings during glowy light, go to: