Some famous lilac collections to visit:

Arnold Arboretum
Jamaica Plain, Mass.

Bickelhaupt Arboretum
Clinton, Iowa

Brooklyn Botanic Garden
Brooklyn, N.Y.

Holden Arboretum
Kirkland, Ohio

Morton Arboretum
Chicago, Ill.

Royal Botanical Gardens
Hamilton, Ontario

Hulda Klager Lilac Gardens
Woodland, Wash.

Cornwall
Ontario, Canada (Cornwall styles itself "the City of Lilacs.")

Highland Park
Rochester, N.Y.
(Rochester bills itself as "the Lilac Capital of the World." Its two-week lilac festival does outlast the Arnold Arboretum's Lilac Sunday. Rochester, N.H., by the way, calls itself "the Lilac City." You can see it's a cutthroat world out there at bloom time.)