Old English mōnandæg preserves the moon in an everyday date. Middle English carried the word onward.
The component display uses base forms; the compound contains a linking ending.
Sources
- Merriam-Webster: Monday
The Old English compound and its related base forms are identified.
- Merriam-Webster: weekday histories
The article explains the moon association and Latin parallel.
- Online Etymology Dictionary: day
Also built on Old English *dæg* "day": holiday, daisy