Its the day when all farmers should be done harvesting and autumn officially begins according to "Bondepraktikan" [1], which says to be done by St Bartholomews day.

Like many old traditions the reasons have for many become lost to time, and now it's an accepted fact that that's the magical night to get some wormwood.

[1] https://en.m.wikipedia.org/wiki/Old_Farmer%27s_Almanac