Because it takes time for your ballot to be processed by the United States Postal Service (USPS), the USPS suggests you mail back your absentee ballot <strong>no later than</strong> one week prior to the deadline.  For Alabama voters, this means you should mail your absentee ballot back no later than Monday, October 26.
Keep in mind that in Alabama you will need at least $1.20 in postage on your ballot return envelope in order for it to not be returned to you for insufficient postage.