According to the Terms and Conditions on Cingular's website: "Nights and Weekends: Nights are 9:00 p.m. to 6:00 a.m. Weekends are 9:00 p.m. Friday to 6:00 a.m. Monday (based on time of day at switch providing your service). Included long distance calls can be made from the 50 United States, Puerto Rico and U.S. Virgin Islands to the 50 United States, Puerto Rico, U.S. Virgin Islands, Guam and Northern Mariana Islands. Roaming charges do not apply when roaming within the service area of land-based networks of the 50 United States, Puerto Rico and U.S. Virgin Islands. International long distance rates vary. Additional charges apply to services used outside the land borders of the U.S., Puerto Rico and U.S. Virgin Islands."
http://www.wireless.att.com/cell-phone-service/legal/plan-terms.jsp NOTE: if you make or receive a call during the daytime rate, it will be billed at the daytime rate, even if the call extends into the nights and weekends rate. Example: If you receive a call at 8:59 and talk for 60 minutes, ALL of those 60 minutes will be billed at the daytime rate. [see http://forums.mobiledia.com/viewtopic.php?p=258401#258401]