The third Sunday of April, August and December from 1800 UTC through 2359 UTC. http://www.arrl.org/rookie-roundup

The second full weekend of September. Begins 1800 UTC Saturday and runs through 0259 UTC Monday. http://www.arrl.org/september-vhf

Third full weekend of August and September. Operations may take place for 24 hours total on each contest weekend. Each weekend begins at 6:00 AM local Saturday though 12:00 midnight local Sunday. http://www.arrl.org/10-ghz-up

  http://www.txqp.net

Three full weekend 48-hour periods (0000 UTC on Saturday through 2359 UTC Sunday). http://www.arrl.org/eme-contest