Norwegian is a budget airline, so their flights are best when you're able to make due with a carry-on and avoid all the little things airlines do to nickel and dime you out of a cheap vacation.
Flight prices change pretty quick, so move on this one if you want it. Otherwise for around $30-40 more you can find other date options, which are still a pretty good deal.
There are some decent prices available from other departure cities in this same time frame as well. Taking off from San Francisco, you can grab round-trip tickets for just $369. From Miami tickets are going for $406, L.A. and Orlando have tickets for $439, and for $455 you can snag tickets out of New York City.