So, on Friday 13th a tiny black cat called "Friday" shows up in towns. Everyone knows they give back luck, so killing them will spawn a JackO. If you wait to kill them on the 14th, no JackO will spawn. Friday also has some cool drops (imo), but mostly Halloween stuff.
The code was backported to TMWA, but I am always amazed at TMWA's skills of breaking stuff (even tested ones); I don't think the JackOs will spawn.
(The cat is also aggressive on TMWA, and won't drop anything because CSV is a pain to manipulate, specially when you are rushing to meet a deadline).
Event concepts by
Hocus
(via telepathy to Jesusalva, isn't he amazing??)