The Skagit Tulip Festival runs from late March to late April. But early April is the best time to see the tulips, fields and fields and fields of tulips, like a little Holland. By mid-April, the tulips have all been 'topped' (cut to be sold), so you'll see a sea of green instead of red, pink, yellow, purple... However, the farms keep a few "demonstration gardens" alive for tourists who come too late. Try to go on a sunny day, your photos will turn out better.
After spending the morning taking hundreds of photos of tulips, drive back into town and look for a free public parking lot. Then walk to the Street Fair (Saturdays and Sundays). Lots of food, goodies for sale, and musical entertainment.