Knowing when the frost dates are is very important. Some plants can' take the cold. But there is still a chance of frost, so some kind of backup plan is important.
If there is a possible frost you can cover your plants with something to help keep them warm.
An aquarium works really well as a mini greenhouse, but even a flowerpot or paperbag will due in a pinch overnight until it warms up again.
You are just trying to capture the ground warmth and keep it around the plant until the sun comes up and warms things again.The paperbag trick is used by growers to blanch growing plants like celery.
