You can check your version in the main Switch menu by hovering over Animal Crossing: New Horizons and pressing the + button. Then, You can read the version number right below Animal Crossing: New Horizons. It should say Ver. 1.2.0. as shown below. If it doesn’t then that’s probably your problem.
If your game is correctly up to date, Isabelle should have started your day with a message about Nature Day and possibly a message about a “suspicious character.” However, you won’t instantly get Redd if you don’t meet certain requirements.
If you did not get that message from Blathers (the earliest you could have gotten it was on April 22 after the update) Redd will not show up. However, once Blathers mentions the expansion, you should see Redd roaming your island the next day.
An easy way to check and see if Redd even showed up is by going to the small beach on the north side of your island. You should see his boat. You won’t be able to enter until you talk to Redd. He’ll be somewhere on your island.
You can still visit other islands to buy shrubs and flowers from their Leif. Each Leif sells different flowers and shrubs anyway, so we recommend shopping around if you can. You can also still get Nature Day Nook Miles tasks without Leif appearing on your island so long as your game is up-to-date.
After updating, talk to Tom Nook and he’ll let you know about Nature Day! One IGN staff member did not have Leif and was still able to complete Nature Day tasks. You can see all Nature Day tasks and rewards on the Nature Day page.
You can also try time traveling back a day by adjusting the system clock, opening the game to register the day change, then return to the current day to see if you get the Nature Day announcement and Leif. We have not tested this but it is an option for you to try. You can learn more about time travel in the Time Travel Guide.
We’ll keep updating this page as we find more issues and workarounds. Let us know what you’ve discovered in the comments.