I'm sorry, this is such an incredibly minor thing, but yet I can't help but want to point it out: you forgot to italicise "The Addams Family" in the current version of this entry. I mean, you must have, as you usually make sure to italicise the titles of books, films and television shows. :) [noted -ed]
This is a name that my grandmother gave to her dog, because there was an annual festival in her town, and people who went were called "Festers". So, in that case it meant "dog who goes to the festival", hahah. But while I can see how this name works as a nickname for Silvester, I don't think it's a good idea for English-speakers because of its English meaning.
To Neko and Akito: yes, and yes. In English, that is what "fester" means. I'm already not a fan of Fester (or Silvester, for that matter), but the reference to rot is yet another reason why I would never use this name. No matter how much I may love Uncle Fester. :-)