Seemingly no-one has mugged up on the book of Esther in the past two days -- time, I think, to wrap the question up.
That which is commemorated (
@GRALISTAIR: please let us know, if you reckon me to have got anything wrong in the following) is reckoned to have taken place around the 5th century BC, in the Persian Empire. Many Jews lived there then, in exile and captivity but, that aside, mostly quite decently treated. Things went wrong for them, when the royal vizier of the then King Ahasuerus -- one Haman, seemingly a very "unsatisfactory person" as the man said -- conceived, for basically trivial reasons, a hatred toward Persia's Jews, and sought to have them exterminated. He managed to get the king's OK for this project; after which -- the "casting lots" bit -- he did that ritual to determine a propitious (for him) date on which the atrocity should be carried out. Fortunately, his plan was thwarted and he himself came to a bad end; the Jews have, re this episode, ever since celebrated the feast of Purim with thankfulness and rejoicing.
@GRALISTAIR: in your main post, you refrained from answering: feeling it -- if I'm correct -- unfair on potential others, to deploy your special knowledge of the matter; but with no factual answers from anyone else, I feel that you've earned the right to set the next question. Should you choose to continue your abstention; I reckon it's open floor.