Look at the orbital path calculator at http://stereo-ssc.nascom.nasa.gov/comet_ison/, around the end of December 2013. Look at the close fly-by. However, it is pointed out that ISON is coming at a steep angle and the site states that it isn't so close.
According to this article, http://globalrumblings.blogspot.com/2013/09/comet-ison-danger-to-ea..., "On closest approach, the comet will pass about 0.0724 AU (10,830,000 km; 6,730,000 mi) from Mars on 1 October 2013, and it will pass about 0.429 AU (64,200,000 km; 39,900,000 mi) from Earth on 26 December 2013."
So, it will pass further from Earth than it did to Mars, but Earth will go through the debris trail soon after it passes.
It also says, "Astronomers say it is a new (strong) comet coming from the Oort cloud which has never before slung around the sun, and therefore has never been weakened by the heat of sun like other periodic comets."
Also, this comet is generating a lot of dust. Could it be enough to create what's known as a nuclear winter and affect weather and crops?
Listen to McCanney's C2C interview at https://www.youtube.com/watch?v=Drf3JeamaYY and read the rest of this document.