There is a bug regarding this issue that is now in review
https://world.episerver.com/support/Bug-list/bug/CMS-9632
A workaround is to add "[UIHint(UIHint.Legacy)]" attribute on the property.
[UIHint(UIHint.Legacy)]
public virtual Weekday ValidDays { get; set; }
That was breaking change that we forgot to document. All properties that use LegacyEditorDescriptor have to be annotated with this UIHint (except Weekday property, where the legacy editor will be available without UIHint).
We will also update the breaking changes document for EPiServer 11.