I invite everyone who has been to the US National Arboretum to mention what was in bloom when they were there. This is likely listed on USNA's website, but having a list here is helpful too for prospective visitors.
I have been most recently on two consecutive KY Derby weekends (lucky me!) in 2009 and 2010, first weekend of May. The Viburnum collection was in full tilt bloom.
I imagine that this genus gets started in mid April (at the latest) and runs on into June, with peak from late April - mid May.
That would be super- having a bloom or 'must see' list by time of year. The Arboretum has this one rough guide http://www.usna.usda.gov/Education/blooming.html
but it's not all inclusive. (I wanted to see and smell the Corylopsis but it had not popped yet.)
Somewhere in the site I read that the visitor Center will locate any particular plant and tell you where it is in the collections.
View from Mt Hamilton (big hill Hamilton) towards Capitol
A path in the Asian Valley
View of the Columns and part of the Meadow ( if not highly decorative, they are novel and historic. Something for kids to marvel at and run around.)