The season runs about Oct 1–20, and it varies year to year; this page's reports say what the road shows this season.
Which exit for larch season on I-90?
From the road: Gold Creek Pond and Hyak (exit 54); Snoqualmie Pass summit trailheads (exit 52); Kachess Lake (exit 62); Cle Elum to the Teanaway (exit 85).