On Sunday, March 12, 2015, most of the U.S. will switch to “Daylight Time” from “Standard Time,” most, but definitely not all of the U.S. Arizona, except for the Navajo Nation, Hawaii, American Samoa, Guam, Northern Mariana Islands, Puerto Rico and the U.S. Virgin Islands don't switch to “Daylight Time” in the spring, and back to “Standard Time” in the fall. They stick to “Standard Time” year-round.
The switch between “Daylight” and “Standard” time doesn't universally occur across the globe. More than 100 countries never change to “Daylight Time”, and more than a few countries switch between the two on different dates than other countries.
In Mexico the switch between “Daylight” and “Standard” time can be very confusing for travelers. Mexican border cities near the U.S. typically swap their “times” on the same dates as the U.S., but the remainder of Mexico changes in April and October, not March and November.