Everyone's worried about the trade deadline, but the real deadline is June 30. That's the deadline for luxury tax purposes (according to my understanding of the CBA).
So while everyone thinks the C's need to make a deal by the trade deadline if they're going to avoid the tax this year, I'm under the impression that they can trade any player not on an expiring contract (so Bird, Yabu, Semi, Williams) any time after their season ends but before July 1 to get below the tax.
Theoretically if they want, the C's keep Yabu for playoff depth, then trade him and Bird (and/or Semi, Williams) on June 30th for a heavily protected 2nd round pick, and none of their salary would hit the C's books for the 2018-2019 season. So no rush yet, this should be easy trade to make if they have to.
Also on top of this, at this point (late June), the C's will have a better idea of what's going to happen in July. Hopefully Kyrie will have reconfirmed to Danny he's re-signing, plus conversations with the Pelicans will be more concrete, as everyone will know where the picks will land, and C's don't need to be scared about discussing their players. The C's and Pelicans could even announce a trade on June 20th (draft day) that will be officially completed as soon as possible (after July moratorium, similar to how Pierce/KG trade was announced June 28, but wasn't officially completed until Jul 12). At this point the C's will know if they need to keep Yabu (or others) for trade purposes or not.
A couple of notes on this per the CBA faq, though not spelled out specifically:
"When the trade deadline has passed. Teams are free to make trades again once their season has ended4, but cannot trade players whose contracts are ending or could end due to an option or ETO."
"Playoff teams can trade players during the playoffs who are not on their playoff roster."
"When a player is traded, his full season salary is transferred to the team salary of his new team, and he counts $0 toward his old team's team salary."
edit - Hey saltlover's back!