Two constraints, and they point the same way.
The light
Overhead sun puts hard shadows in eye sockets and under noses, and blows out anything pale. It is the least flattering light there is for a portrait, and Chennai has a great deal of it for most of the year.
The hour after sunrise and the hour before sunset give you low, warm, directional light that wraps around a face instead of carving into it. That is not a stylistic preference; it is most of the difference between a good outdoor portrait and a squinting one.
The heat
The other half. Nobody looks relaxed at 33°C with high humidity, least of all someone in a maternity gown or a toddler in a costume. An hour outdoors at midday produces tired, uncomfortable people in every frame.
Sunrise sessions are cooler, quieter and — at popular locations — dramatically less crowded.
What this means for scheduling
Outdoor sessions start early. Uncomfortably early: often 5:30 or 6:00 AM at the location, which means leaving home in the dark. Every couple we suggest this to resists it, and every one of them sees the difference in the images.
Evening sessions work too, with a narrower usable window and more people around.
Practical points
Bring water. Bring a change of clothes if the session runs long. For babies and toddlers, bring more than you think — heat makes a session end early far more often than boredom does.
The monsoon
Between roughly October and December, outdoor sessions get moved at short notice. We do that at no cost, because a compromised set of images is worse than a rescheduled one.
Overcast days, incidentally, are excellent for photography — soft, even light all day. A grey morning is not a cancelled session.