Help, I've mixed my week up!
Pyth, 7 bytes

o%CN258


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Convert each string to a number via treating its ASCII codes as a base 256 number, then take that mod 258, and sort. This gives the mapping

['Monday', 49]
['Tuesday', 75]
['Wednesday', 89]
['Thursday', 99]
['Friday', 103]
['Saturday', 125]
['Sunday', 211]


Same length but less fun is

.P1314S


The 1314th permutation of the sorted input in lexicographic order.