At two places in the rrule documentation (http://dateutil.readthedocs.io/en/stable/rrule.html), we have:
"Must be one of the MO, TU, WE constants"
"one of the weekday constants (MO, TU, etc)"

Is it better to at least have one systematic reference to weekday constants - a table, maybe, towards the beginning of the documentation?

Monday(MO)
Tuesday(TU)
Wednesday(WE)
..

Its probably obvious, but never explicitly mentioned(in the rrule documentation) that we require the first two letters of the weekday to refer to that day.