Lavoisier

No more needed, now it can be done with EXSLT:

<connector type="StringConnector" xmlns:date="http://exslt.org/dates-and-times">
    <parameter name="content" eval=" 
        choose(date:day-in-week(path('date'))=7,
            date:format-date(date:add(path('date'),'P2D'),'yyyy-MM-dd'),
            choose(date:day-in-week(path('date'))=1,
                date:format-date(date:add(path('date'),'P1D'),'yyyy-MM-dd'),
                path('date')
            )
        )"/>
</connector>