Feast of St. George. A ubiquitous image in the Balkans, the slayer of the dragon. (Although, in the Golden Legend, apparently the beast's life is preserved long enough to lead it into the town for a ritual killing, as opposed to killing in struggle.)
A message to their children, perhaps: Dragons be here.
Associated with this date since the 13th c., the feast proclaimed at a synod at Oxford.
https://www.newadvent.org/cathen/06453a.htm