Okay, so it isn’t on Whidbey Island, but the Skagit Valley Tulip Festival is just a few minutes off-island, and is a must-see for anyone who is venturing this way. It takes place during the month of April each year. The Daffodils are in full bloom right now, although with the wet weather this year, the tulips are blooming a little late, and I expect they will continue into early or mid-May.
The Roozengaarde Nursery has a great interactive map that will let you see the status of blooms in each of the fields, and if you click on the flower at each field you can see what variety of tulip is planted there.
A great day trip from the Seattle area is to come north on I-5 to Conway, travel through the Tulip fields, and then head west to Whidbey Island via Deception Pass Bridge. Take SR 20 south to HWY 525, and then home on the Mukilteo Ferry. It makes for a fun day, with lots of great sights to see along the way.