It doesn't happen all at once. I have photos taken in December that feature flaming foliage that has held on since October . I have 1 Japanese Maple that hasn't a leaf on it , and one that has just started to turn. The Crepe Myrtle in the back garden is red-orange-gold , the front garden specimen is still green.
These photos were taken on Saturday morning a bit south of Calistoga and at Trefethen Vineyards