This site describes solve-engine as it is on main: 2.43.0, which npm does not have yet. npm installs 2.40.0, so a page may show an answer that version does not give yet.

WeekdayName

type WeekdayName =
  | "monday"
  | "tuesday"
  | "wednesday"
  | "thursday"
  | "friday"
  | "saturday"
  | "sunday";

Defined in: packages/engine/src/calendar/WeekShape.ts:21

A day of the week by name, as the configuration spells it.