I think the Examiners are simply blinded by Hubris. Every one points out how foolish it is to let these high-potential people get killed off in an exam but they're doing that because they really do believe those people are expendable. The Demon King was slayed and now they're confident nothing...
I think Grandma is referencing Walpurgisnacht (Walpurgis Night), basically halloween in the spring. From Oxford Phrase and Fable: "Walpurgisnacht Night (in German folklore) the night of 30 April (May Day's eve), when witches meet on the Brocken mountain and hold revels with the Devil..."