E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Saturday. A contraction of Saturni-dies. In French, m and n are interchangeable, whence Saturne is changed to Saturme, and contracted into Same. M. Masson, in his French etymologies, says it is Sabbati dies, but this cannot be correct. MARDI is Martis-dies, VENDREDI is Veneris dies, JEUDI is Jovis-dies, etc. (The day of Saturn, Mars, Venus, Jove, etc.)