I don't know either, but I do know that on these days I'm more prone to the kleshas of anger and associated emotions. I can find phenomena more prickly on these days. I think there is something about that energy associated with these practices on these days. But you'd have to ask a knowledgeable lama.
"When a Dzogchen Yogi hears Shakyamuni Buddha turning the Wheel of the Dharma of the Four Noble Truths he hears Samathabhadra proclaiming the most profound Dzogpachenpo." - Nyoshul Khen Rinpoche