Hopefully before the actual release date, would suck downloading on the day of release.
If you are on Playstation and you order via the PSN Store, you can begin preloading on September 7th. If you are on an Xbox, you will have to wait until the game releases.
If you are XBox One and plan Digital you may want to check couple days early. Started to see games open for pre-download but nothing official at all for Destiny when you can start Downloading. Assume it's the 9th but you can always check couple days early to see.