This is gonna get long, but hear me out.
I think Animal Crossing Switch is going to released sometime between the summer and early fall season, but after E3 (late Q2/early Q3 it's called, I think?). Why? Well for starters...take a look at this
article.
Don't worry, these guys are pretty reputable, and typically report on things after they've happened. Anyways, tldr: the article talks about Nintendo's financial business, and reconfirmation on upcoming 2019 games. Scroll down and see the games list for 2019 release. Notice the first three have specific dates/release periods, but everything else just says 2019. Notice also that the 1st party big release games are scheduled for 2019 spring, and late fall/winter 2019/early 2020, kind of like what happened late last year/early this year with SSBU. Bayonetta 3 and MP4 are both TBA, so we can assume those are still in very early development, and we probably won't see them this until much later this year, if we see them at all. But everything else, says 2019. We already got SSBU for winter of 2018/2019, so that's done. And clearly Yoshi and FE got Spring covered. Mainline Pokemon, enough said, that's gonna be THE IT game of late fall/winter 2019. Everybody wants that. I want that.
So Nintendo already has all their first party bases covered for the rest of the year. That just leaves summer. Luigi's Mansion and AC are both hotly anticipated first party games. Knowing Nintendo, they're not just gonna release two hotly anticipated first party games without showing them off, and the biggest event to show off your stuff when you know everyone wants them and you know they're that good? E3, of course. Also knowing Nintendo, they're not gonna release two first party games at the same time, especially when those games appeal to a very similar audience, because Nintendo doesn't want to compete against itself. They're made of money after all, so rather than having you choose between one or the other, they want to you buy BOTH. E3 2019 is from June 11th-13th. I don't know which one is gonna drop first, but if I had to guess, I'd say it's Luigi's Mansion 3. I mean...Luigi is a Mario Brother, after all. Everyone knows him, loves him, and wants to play his game. I know I do. Then, maybe a month or few after that, AC will drop. I don't think AC will come in the late fall, because it'll be too close to Pokemon. And like I said, Nintendo wants you to buy all their games, not pick and choose.
So that just leaves late summer or early fall...personally, with the way they're doing things, I think it'll be early fall. Assuming that LM3 drops first in early-to-mid summer, a lot the target audience is going to snap up the game immediately like they always do so the initial hype will have worn off by then, and people will be looking forward to something new. And since Pokemon will have late-fall/winter covered into the holiday season, I doubt it'll come in that late in the year.
Anyways, that's just my opinion, based on the info that's available. Outside from Nintendo saying so, there's no real way to tell, so I'll just be waiting for them to release whatever info they have, just like everyone else.