The Weekday of Month function is supported, returning an integer 1-5, nth occurrence of day of week in month. When dragging a Weekday of Month into a variable, the following tree element is created.
Variable Creation > Input > Variables: SQL Elements pane - Calendar > Weekday Of Month
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 Weekday of Month function.