See info here.
Quick overview: UPDATE 12/13: MEETING SPOT APPARENTLY NOT IN THE PARK, MEET ACROSS FROM THE ARCH ALONG FIFTH AVENUE AT Washington Square North/8th Street up til 12th Street. (Not sure how this is going to work but this is what I have been told…)
Further update 12/14: In the end, thankfully the march began IN the park (although some may have moved along Fifth Avenue – as this late notice I received directed – which was where the march proceeded). It was nice to see the park used for this purpose again – protest. It was somewhat funny (?) how the march went through the Arch filing around the Christmas tree! I will try to run some photos.