The prophecy about 3 days of darkness, or really any length of time in which there is total, absolute darkness on the planet is a prophesied event drawn from Islamic teachings and tells of a time when 3 days of darkness will come upon the world that will act as a final warning to all "infidels" (ie, non-muslims) to either convert to Islam or die.  It is a clearly false teaching taken directly from either the Koran or the Hadith.  Strangely, this prophecy has been taking on a life of its own lately, morphing from its original version (3 days) to five days, seven, fourteen, a month, a year, three weeks, six weeks, you take your pick.

There's also repeatedly been dates set for when this prophecy will happen which have, continuously and repeatedly, failed to materialize.  This prophecy is also very unbiblical as it is clearly evident in scripture, especially due to its glaring absence, that such an event will not happen.  The closest approximation and similitude is the time of absolute darkness that will cover the beast's capital city (ie, antichrist) just before it is ultimately and finally destroyed.  However, no specific timeframe is given for this, although many have theorized just how long that'll be.  Even so, this global 3 days of darkness, or however many someone wants to declare, is a false prophecy.

Also, various "updated" modern versions of this prophecy have been released by so called "prophets" of God, and numerous eastern religions across the world.  However, they all point back to the original Islamic prophecy from which they were spawned, even though they've been dusted off, polished up, and given new retellings of the original.  Either way, it's all the same false, heretic prophecy.