O(n^2) time because String.sub creates a new string for each element of the input

Changing Array#include is not an ideal solution as it may break other code if thrown into a more complex program.

This is not an issue, it's a suggestion.

shouldnt the second part of the regex for months w 30 days be 30 rather than 3[0-1] ?

the two functions are pretty repetitive in their solutions. my advice would be to narrow the kata down to one function.

if you look at the characters of 1875, the biggest symmetrical number in it is simply 8 because every character is unique.
since 8 is the biggest character, it is also the biggest palindrome.