The Day of Week function is supported, returning an integer 1-7, the number of the day within a given week assuming 1/1/1900 Is Monday. When dragging a Day of Week into a variable, the following tree element is created.
Variable Creation > Input > Variables: SQL Elements pane - Calendar > Day Of Week
Columns and/or other expressions that resolve to a date can be moved over the (empty) branch of the tree.
There are no special properties for the Day of Week function.