Red Oaks (Quescus spp.) are one of the few deciduous trees in MN that hold onto to their leaves throughout the winter. So when do they drop them? The short answer is "In the spring". But I'm curious and would like to know with a bit more precision, so I intend to keep an eye on a couple of Red Oaks in the following weeks to get more details.
Another related question is "Why do Red Oaks hold their leaves through the winter." It could be that the leaves provide some protection for the buds against the cold. Or the leaves drop in the spring to suppress the germination of other seedlings. Or so they can provide a compost layer in the spring when it's most useful to the tree. I'm not entirely convinced of any of these ideas.
An idea I like better is that the Red Oaks are somewhere in the middle in the evolution of the deciduousness (probably not a word, but I like it, sorry). All trees loss leaves, just some do it more gradually, namely the confers (except Tamaracks, which are deciduous conifers). The deciduous life style is a a relatively recent development in terms of the evolution of plants. Perhaps the Red Oaks exhibit an intermediate form of it, somewhere between the more ancient conifers, and the rest of the relatively newer deciduous flowering trees.
The habitat of holding onto dead leaves, or other withered parts is termed marcescence. The related American Beech (Fagus grandifolia, both in the Beech family, Fagaceae) also show marcescence. Ironwoods (Ostyra virginiana) are also marcescent, but unrelated, being in the Birch family, Betulaceae.