Aurora / northern lights / arsarnerit have been super active in Sept and Oct month to date.
I've been seeing posts all over about the auroras being nice and vivid. I really suggest September as a good month to visit Greenland. This year it was getting a little snowy all over by the back half of the month.
Generally speaking, I look at the forecast for a KP3 or higher but really if the weather is not cloudy and you can find a dark place, just bundle up and go out.
My biggest problem this year was that I was too far north (Qaanaaq is too far) or the clouds blocked the view. Weather is a big determinant.
There aren't that many dedicated aurora tours in Greenland yet but one of the better ones was in Ilulissat with Disko Line Explorer on the boat and then I used to go out by Hotel Arctic in Ilulissat. Kangerlussuaq is great but the likelihood of getting stuck there is too high right now.
I hear there are good dark spots in Nuuk like near Inuk Hostel as well. On cruises, I see them nearly every night as long as the sky is clear enough and if I wait long enough. They might not be that vivid though.