I found these helpful on Visa.com -

Receive one FREE day when you reserve a midsize through full-size 4-door car in the United States, Canada, Latin America or the Caribbean for a weekly rental when you pay with your VisaŽ card.

To redeem this offer, click the link below or call 1-800-GO-ALAMO and request ID# 7000799 and Coupon Code FK8B at time of reservation.

Valid: 01/01/2006 to 06/30/2006

Valid for a rental of at least five days.
Valid through 6/30/06.
Call Alamo at 1-800-GO-ALAMO or book online at alamo.com. Be sure to request ID# 7000799 and Coupon Code FK8B at time of reservation.
One coupon per Alamo rental and void once redeemed.
Original coupon must be presented at counter upon arrival.
Free day is prorated against base rate for entire rental period, which does not include taxes (including VAT/GST), governmentally-authorized or imposed surcharges, license recoupment/air tax recovery and concession recoupment fees, or optional items.
Offer is subject to standard rental conditions.
Blackout dates may apply.
Not valid with any other discount or promotional base.
Subject to availability and valid only at participating Alamo locations.
24-hour advance reservations required.
Offer not valid in Manhattan, N.Y., or San Jose, CA.
Void where prohibited, taxed or restricted by law.
Applicable taxes must be paid by bearer.
Offer valid only when you pay with your Visa card