Info from my expertise in autumn-ness (read: I googled)
Autumn equinox is held on a day when the day is exactly as long as the night. Thus (and for some unexplained reason, this doesn't apply to spring?) autumn is the season of balance, and balance is celebrated. The autumn festival also celebrate home, hearth, loyalty, equal exchange, and friendship based on equality/brotherhood.
Autumn is also about celebrating achievements, reaping the rewards of our harvests, ect. It is the time of fruition; a time when long-term plans first yield results that will carry one through.
In norse mythology, the festival Disablot (sacrifice to the Disir, or Feast of the Disir) pays homage to minor female fertility deities associated with birthgiving, and protecting home and crops.
In eastern cultures, the Moon Festival is celebrated at this time, which has many varying legends of an immortal couple, which give different explanations on why the moon is the brightest at this time of year. Lots of good source material there to google.
Psychologically and literary, autumn is often associated with depression, wistfulness, loneliness, and yearning.
So... more than death or decay (which I think of as Winter's province) autumn is a time of balance and celebration where the joyous abandon of summer has become tempered by the knowledge of winter's approach, giving rise to a deeper appreciation for what we are given.
Now, how to turn that into magic? Er, i dunno.