Here is a compendium of the info on this.
IMO, it matches the description given by John in Revelation 12:1-2, and there is no other time in the heavens that fits this description as precisely (or more) to John's description as this one does.
The popular prophecy people are flat-out calling this false without actually addressing the alignment itself as it relates to the text. There has been some attempts to claim it isn't unique, but those have been pretty well refuted. The thing about this is that it is the convergence of several astronomical events that individually have repeated throughout history; but being all together, is what makes it a unique event in human history. I think GST is correct in the motivation behind this for some - there is an incentive to call this false and "speculation" because it actually removes speculation and cuts into their business of selling books and videos speculating how the end times will happen. They'll all have egg on their faces also, because this does not conform with any popular preacher's end times timeline chart.
There is still a question as to what the Sign of the Dragon is in Rev. 12:3-4. The possibilities is that it relates to the constellations of Draco, Serpens, and/or Hydra, the red giant stars of Antares or Arcturus, or a yet future unexpected celestial event, which is strongly suggested (IMO) by the use of "behold" in verse 3, but not in verse 1-2. The Greek word for "behold" is used to call attention to something (like we say "look,...."), or it can call attention to a unexpected or seemingly impossible event (as we say "look!!!").
Some are taking this as a rapture sign. If we are to take Revelation 12:5 to mean the church is the child here, then the Rapture is pre-trib, given the seven years the Woman is on the run (which is the 70th week of Daniel)as described in Rev 12:6 and 14. This chapter has been historically the primary passage in support of Pre-trib. Read the compendium link on how the justification for this is linguistically in the text.
IMO it doesn't matter if nothing else happens on Sept. 23rd, the fact is that the Great Sign given by John in Rev. 12:1-2 is present in the heavens, as he described it, on Sept. 23rd 2017. That in itself is a fulfillment of prophecy that should excite people looking for the return of Jesus - whatever else that happens during or after that is part of the only timeline chart that matters: God's.