Revision bd65df3d-ffde-43cf-9597-a514a1cbc8f6 - Code Golf Stack Exchange

## Ruby 1.9, 85 characters

 f=->a{require"date";d=Date.parse(a,0,0)+1;d+=1until d.day*d.month==58;d.strftime"%A"}

Straightforward solution. Call the function with `f[args]`.

* Edit: (87 -> 97) Fixed the `0001年01月01日` testcase.
* Edit 2: (97 -> 91) Date.parse allows specifying the date of the calendar reform as well.
* Edit 3: (91 -> 87) Use a lambda instead of a function. Thanks [Dogbert](https://codegolf.stackexchange.com/users/308/dogbert)!
* Edit 4: (87 -> 85) Remove unnecessary spaces. Thanks again, [Dogbert](https://codegolf.stackexchange.com/users/308/dogbert)!