Thanks, WAB, for the information about the Mansi being asked about holidays.
I assumed that the Dyatlov group wasn't aware of any particular holiday. I know the Soviet attitude about folk religion all too well: "superstition." (Btw, German immigrants brought groundhog day to the US and Canada). The day is also the Christian feast day of Candlemas, but of course that doesn't exist in the Russian Orthodox church. I found out that the Khanty holidays, the reindeer feast and the bear feast are in March: spring comes later there, so it makes sense. So my suspicions about that day turned out to be Eurocentric.
I've dropped the "time" issue and think that the event is more about "place." And I don't think the Mansi were involved since they had a better rapport with the Soviets, even being used as bounty hunters for gulag escapees in some cases. I think it's pretty clear from some of the Mansis' testimony, though, that they already knew what had happened. If I were to suspect any locals, it would be the Khanty or Ostyuks, especially the five "wild" members of the tribe that one of the Mansis identified as living near the pass: they are the ones who revolted against the Soviets in the early 30s, leading to so much blood shed on both sides that I don't think the death toll will ever fully be known.