Does it have to get that cold overnight, or during the day?

The way I interpret it is if you have around 40 days with temperatures of 40F or below you are good. I've also heard up to 50F is ok. And really that's only in the first couple of months so they can bulb up. Also, they will stop growing once soil temps at around 1" reach about 80-90F depending on variety. I mulch with cut grass to keep the soil cooler.

I have certainly read people being very particular about it and even recommending refrigerating the bulbs before planting, but I've never worried about it too much and have always had good results, even planting quite late for my area. As is often the case advise is either very generalized or very specific to a variety and set of conditions. Give it a shot!