Default value of nullable enum (Nullable / enum?)

Question

What is the default value of a nullable enum (Nullable<enum> / enum?)?

public enum Days
{
    Sun,
    Mon,
    Tue,
    Wed,
    Thu,
    Fri,
    Sat,
};

What will default(Days?) evaluate to?

Intentionel duplicate of What is the default value of the nullable type “int?” (including question mark)?. If you don't know that nullable integer (int?) acts the same as nullable enum (enum?), you do not know that the question is the same, and therefore do not read the question nor its answers.