Is the lychee as cold tolerant as mandarins and satsuma oranges? Can they survive a freeze to 20 F with 12 hours below freezing?

According to urbanharvest, a organization in Houston, they're hardy down to 25 degrees (though who knows for how long).

http://www.urbanharvest.org/advice/fruitgardening/fruitvarieties/lychee.html

As far north as I've seen them is in zone 10 Merritt Island, Florida. They were mature trees. They grow mangoes there as well.

Mine held out the cold pretty good. 28 degrees for like 3 hours and a few nights below freezing. This was my experience this past winter. They weren't phased at all with those temps. The same cold defoliated my mango trees.
All inground btw and about 2 year old trees

They're similar to lemons -- will take a couple degrees of frost. But definitely less hardy than grapefruit, and far less hardy than oranges or mandarins. The lightest frost will singe the leaves. We've lost major limbs here in Lakeland Florida, when citrus showed no damage at all.