It is mustahab (preferable, praiseworthy) for men to perform fajr when there is more brightness in the sky. For women it is mustahab when it is not yet bright.
In summer zuhr salaat should be performed at a time when the day is cooler, and should be hasten in winter, except when it is overcast.
It is permissible to delay the asr as long as the sun does not change color (becomes yellow). To delay more than this is highly disliked. In overcast conditions asr should be hastened.
Maghrib should also be hastened except when overcast.
Isha is to be delayed up to the first third of the night but hastened when overcast. To delay until midnight is permissible but after this it is highly disliked.
To delay the witr to the last part of the night on the condition that one is certain of being awake at that time.