Some vegetables are chilling sensitive. Soluble solids make vegetables freeze at lower temps than water. Successful postharvest handling cannot be taught by following a set of rules because there are so many continually changing variables. You need to experiment within your own system to learn what works best for you, and train yourself to notice parameters such as temperature, humidity, wind, crop temperature, ventilation, and rough handling. List of resources is attached to this handout.
Produce is coolest just before daybreak.
If you can't harvest in the early morning, do it in the evening, or when it's cloudy. Less radiant heat so less field heat. Slower metabolism so less vital heat. Prevent wounds, abrasions, bruising, or punctures at harvest. Bruising from dropping the produce or dropping something on the produce.