Comment # 2 on bug 70693 from Patrick Ohly

It turned out that the Synthesis engine lacks support for RRULEs with
BYDAY/BYMONTH values including a number (e.g. "first Monday"). It only supports
BYDAY=MO, not BYDAY=1MO.

I'll keep this open as a feature request but don't have time to work on it
myself at the moment - sorry.