The current rule
Since the Energy Policy Act of 2005 took effect in 2007, daylight saving time in the United States begins at 2:00 a.m. local time on the second Sunday in March and ends at 2:00 a.m. local time on the first Sunday in November. Because the change happens at 2:00 a.m. in each zone separately rather than simultaneously nationwide, the country spends a few hours with its usual offsets scrambled as the change rolls west.
Which states opt out
Hawaii and most of Arizona do not observe daylight saving. The Navajo Nation, which extends into Arizona, does observe it, while the Hopi Reservation inside it does not — producing one of the more famous timekeeping oddities anywhere. The territories of Puerto Rico, the US Virgin Islands, Guam, American Samoa and the Northern Mariana Islands all stay on standard time year round.
Attempts to make it permanent
Congress has repeatedly considered ending the twice-yearly change. The Sunshine Protection Act, which would make daylight saving permanent, has passed the Senate by unanimous consent and then stalled in the House more than once. Under current federal law a state may opt out of daylight saving entirely, as Arizona and Hawaii have, but may not adopt it permanently without an act of Congress — which is why state-level permanent-DST laws sit dormant awaiting federal approval.