Testing a grimoire on the Brocken

http://www.atlasobscura.com/articles/the-brocken-experiment-failed-to-change-a-goat-into-a-boy?utm_source=Atlas+Obscura+Daily+Newsletter&utm_campaign=eae8ac97a1-EMAIL_CAMPAIGN_2017_05_11&utm_medium=email&utm_term=0_f36db9c480-eae8ac97a1-64536349&ct=t(Newsletter_5_11_2017)&mc_cid=eae8ac97a1&mc_eid=9337bd4ceb

Neat experiment!  However, the ritual was to take place in the Winter, and this experiment was done in June.  I'm curious why?