The Mölltal and Hintertux glaciers and the summer ski area in Zermatt are already or still open. The Stilfser Joch is probably also open until November, but it's not quite online yet, so it's better to call ahead.
WATCH OUT! As always, the following applies to skiing in autumn: glaciers have holes and where there are no glaciers, there are rocks. We strongly advise against crevasse slalom in 30 cm of fresh snow on bare ice and similar activities and recommend preparing for the season on the piste.
September
Austria
Pitztal Glacier, probably 16.9. (Sun)
Sölden, probably 17.9. (Mon)
Stubai Glacier, 21.9. (Fri)
Kaunertal Glacier, 29.9. (Sat)
Italy
Schnalstal 15.9. (Sat)
France
Tignes 29.9. (Sat)
October
Austria
Kitzsteinhorn 1.10. (Mon)
Kitzbühel 13.10. (Sat)
Switzerland
Engelberg 13.10. (Sat)
Diavolezza 20.10. (Sat)
France
Les deux alpes: possibly open 26.10. - 4.11., then possibly closed again until December (information on website difficult to decipher)