American and Delta are having another fare war to Hawaii. Fares start at $335 round-trip and are available for travel this spring, late summer, fall, and next winter. United has some routes available for last minute travel this winter.
Travel is valid on Mondays, Tuesdays, Wednesdays, and Thursdays, from April 3 through May 24, and again from August 14 through January 2019. Travel to Hawaii appears to be blacked out between December 7, 2018 - January 6, 2019. Not all routes will be available for the entire travel period.
This sale is unadvertised so book quickly if you see something you like. Most third party booking sites will have a 24-hour free cancellation policy so you can think about it for a day.
This is not a complete list. Check flights from your city to see if fares have dropped to Hawaii.