Personally I'm thinking between August and October. We have Mario Maker 2 in June, Fire Emblem in July, and Pokemon will be the big holiday title this year coming in November/December for sure. So there's that window.

It's far too late for them to announce a release in the first half of the year. Only other game we've been promised that's yet to get a release date besides AC is Luigi's Mansion 3. Presuming they both end up releasing according to 2019 schedule without delay, we'll get those 2 games within the August/September/October window. I'm optimistically hoping for August, since Luigi's Mansion seems to suit a fall/halloween release, but who knows? They could both be delayed or AC could come out in October or something.

I doubt we'll get any more info until the next direct or at E3 in June, so we'll be hanging around waiting for a couple months to get any real confirmation. For now all we can do is speculate. I'm trying so hard not to get too excited but I'm already way overhyped!