I think Q2. I think before the end of this year we will get an Animal Crossing Direct and then a release date. I don't think it will be any later than September 2019 (and that is the absolute longest they'll wait) and here's why:
Online Play. They want to get as many subscribers as possible before the end of the first year of launching the service, aka next September. They aren't going to release a game that a lot of people are buying Switches/Online play more than a year after the initial launch of the service. Smash and Splatoon will bring in a lot of Online revenue but Nintendo has something good with Animal Crossing and they know it. Beyond Smash, Animal Crossing was the #2 requested game to come to the Switch. They want this in their first year of Online service sales. I think Summer is perfect because of school being out, so my guess is anytime between May and August.
You also have to look at other game releases. Pokemon Gen 8 will be a huge hitter when it comes to sales, and Pokemon games almost always come out in November or fall in general. Pitting Pokemon vs. Animal Crossing in terms of people who can only buy one for whatever reason (cost, too close to christmas, etc) isn't a smart move.
ANYWAY those are just my reasons for thinking it's Q2. Obviously no one knows for sure and we can only speculate for now.