Yeah, good question. I think there are a couple of possibilities.
First, we can probably rule out that it was just time passing, even though that's something that can happen between chapters. In other episodes, Saul does try to kill David; fails; repents; and things go back to normal for a while. But in this case, we can guess that it was under a month since the spear-hurling, being that the event is the new moon feast. We also notice that only Jonathan thinks everything is fine; if this were like the other episodes, both David and Jonathan might expect the king's favour, whereas here David is still pretty shaken. David has just been in Naioth, and he's still there, having recently had a spear hurled at him.
One possibility is that Saul is counting on the fact that David is still part of the family and things will "heal over". After all, the chapter starts out with Jonathan denying it all and trying to assuage David's fears. Everyone, including David, knows Saul is unstable and tormented; maybe Jonathan is being an enabler and saying "Oh, you know him. He didn't really mean any harm." Saul too must realize that people expect him to lose control and then apologize for it later, and he knows that David's ties with Jonathan and his wife keep him bound to Saul's family.
Perhaps the more compelling explanation is this isn't just "showing up for dinner". It's a feast that the king has invited David to, and evidently he hasn't cancelled the invitation. If the king invites you, you can't just not show up; it's probably at best a courtly faux pas and at worst a punishable offence. David views it this way too: he doesn't say, "I imagine he'll understand why I don't feel like going." Instead, he comes up with an excuse — and he's even forced to make it something so big it's easily falsifiable.
This explanation seems the most likely to me, even if the rest might also play a part. Saul doesn't expect David to come as a friendly houseguest, but to obey him as a subject.
