Very rough guess, I only know of the Festival of the Serpent from folklore, and it fell the 15th to 17th before it was taken over by the banisher of the serpents - St Patrick. But it might have really fallen about the 5th if the changing to the Julian calendar changed the dating.
I also wondered if it was a star alignment, and the move to the 5th might be due to pregression.
Thats just the first ponderings looking at the first on the list...