Mine is using the weekday names in French, which is a bit convoluted but works well:
– Mars => mardi=> かようび=>かせい
– Mercury=> mercredi => すいようび=>すいせい
– Jupiter => jeudi => もくようび=>もくせい
– Venus => vendredi => きんようび => きんせい
– Saturn => samedi => どようび => どせい
by shirokuma_uk