January 29, 2016
Not coincidentally - they're both named after Mercury.
- dydd LLUN = LUNdi (moon; Latin: dies Lunae)
- dydd MAWRTH = MARdi (Mars; dies Martis)
- dydd MERCHER = MERCREdi (Mercury; dies Mercurii)
- dydd IAU = JEUdi (Jove, Jupiter; dies Iovis)
- dydd GWENER = VENDREdir (Venus; Latin genitive: Veneris; dies Veneris)
- dydd SADWRN = SAMEdi (Saturn; dies Saturni)
Only Sunday is not related, being "day of the Sun" (dies Solis, dydd Sul) in Welsh but "Lord's day" (dies Dominica, dimanche) in French.