Passing static array in attribute

No, basically.

You could, however, subclass the attribute and use that, i.e.

class AwesomeFooAttribute : FooAttribute {
    public AwesomeFooAttribute() : FooAttribute(A.Months) {}
}

or:

class AwesomeFooAttribute : FooAttribute {
    public AwesomeFooAttribute() {
        Nums = A.Months;
    }
}

and decorate with [AwesomeFoo] instead. If you use reflection to look for FooAttribute, it will work as expected:

[AwesomeFoo]
static class Program
{
    static void Main()
    {
        var foo = (FooAttribute)Attribute.GetCustomAttribute(
            typeof(Program), typeof(FooAttribute));
        if (foo != null)
        {
            int[] nums = foo.Nums; // 1,2,3
        }
    }
}

You could perhaps nest this inside A, so you are decorating with:

[A.FooMonths]

or similar

Unfortunately this is not possible. The attributes (including the values of their arguments) are placed into the assembly metadata by the compiler so it has to be able to evaluate them at compile time (hence the restriction to constant expressions; the exception for array creation expressions was obviously made because otherwise you could not have array arguments at all).

In contrast, the code that actually initializes A.Months is only executed at runtime.