If you are talking about trillium in west Mich, the first weekend of May is peak. See this...
May 4 & 5

TRILLIUM FESTIVAL PJ Hoffmaster State Park. A celebration in recognition of spring. The event showcases the natural communities & unique species of the sand dunes. Sponsored by the DNR, Gillette Natural History Association and Radium. Saturday 10:00 am-4:30 pm and Sunday 12:30 pm-4:30 pm. This event is wheelchair accessible. The only cost is the Motor Vehicle Pass for the state park. For more information contact (231) 798-3573.

Thanks! That is just what I needed.

Not sure what area on the western side of Michigan this Festival is, but you should hit Meijer Gardens while over there. We finally got to visit last fall, really had a great time. I would allow at least half a day, to view things, maybe longer. Fall had frozen some things, so perhaps warmer Spring would have more to see. There is the inside garden, a couple of movies to see, lots of artwork. Then there are extensive outside garden areas, cover a lot of variety and quite entertaining. Bring good walking shoes!
Certainly worth a visit and the time to look around. Quite a bit of modern art planted around the grounds. Some very cool like the horses, others just neat to look at like the wind sculpture and the eggs by the pond. Some sculpture I couldn't begin to understand!

Here is the site:

http://www.meijergardens.org/

Have a great time.

PJ Hofmaster is near Muskegon. Alss, if you just drive through the Highland Park area of Grand Haven, you'll see lots of trilliums growing wild, too. It's beautiful when they are blooming.