What driving circumstances does the ATF heat up the most?

I understand in peak traffic and during hard driving, but what about highway speeds, or just idling?
And does the ATF heat up even when ambient temperatures are cold or is it just an issue in warmer months?

Biggest source of heat is fluid friction in the torque converter. At highway speeds the TC is locked up, but you will get a lot of heat transfer from the engine.

If you want to solve the "porridge" question Goldilocks, get a trans temp sensor.

Cooling for the ATF is controlled by fluid flow, Load, Air temp and because it is cooled in the water radiator - the engine water temp; so I would guess at highway speed - good fluid and air flow - least load, sitting at traffic lights in mid summer with the A/C on - worst.

I always drop to neutral at the lights - at least the engine isn't fighting the torque converter... and the fluid can flow a bit faster - trade off is the continual shocks to the clutches going into gear...

High load and low revs is the worst.
Full throttle in peak hour traffic, or towing.

The heat exchanger will hold the trans up to engine temps. If you cook the engine, you will cook the trans as well for the same reason.