Usage of その here?

Question

明日から収穫祭だ。 今日はその前夜祭だということで、実質、今日から収穫祭は始まっていると言っていい。

収穫祭 being the festival to which 前夜祭 refers.

Usually I find myself comfortable considering その as a rough equivalent of "that", but in this specific context it doesn't look very appropriate. If I had to guess, this その here looks more like "(Regarding) that", that being the previously mentioned festival, which makes sense to me despite this specific function not being mentioned in the kokugo jiten I own. So I wonder, how does this その work on a linguistic level?

Answer 1

The source of the confusion seems that you misunderstand the meaning of 前夜祭, which literal means pre-festival night or eve festival. Usually it refers to festive activities held at night before the beginning（i.e., the eve) of a festival (e.g. it happens on October 10 when the festival starts from October 11).

As suggested in the comment, その here is its. Thus it means The harvest festival starts from tomorrow, Today is its 'eve festival', so practically we can say the harvest festival already has started today.