Autumn is great for two things in the garden :
1. Making a garden. Masonry, woodwork, planting--it's a wonderful time of year to build and plant a garden. The ground is workable for hardscape construction like decks, terraces, fences, pergolas, and so forth. The cool air makes for easier transplanting for plants.
2. Planning a garden. Late autumn and winter are great times of year to plan a garden. This puts you first in line for construction in Spring!