The second Sunday in May every year is “Lilac Sunday” at the Arnold Arboretum of Harvard University, 125 Arborway, Boston, Massachusetts (map).

Probably because the second Sunday in May also is Mothers’ Day, it’s the only day of the year that picnicking is allowed at the arboretum (of course, if you’re not already in Boston today, Road Trips Gardeners this information isn’t of much use).

Lilacs were among the first cultivated plants grown at the arboretum. You can find remnants of a lilac hedge planted in the mid-19th century by Arboretum benefactor Benjamin Bussey on Bussey Hill. The oldest lilac on record in this collection is a Syringa reticulata (Japanese tree lilac) from 1876.

The Arnold Arboretum’s lilac collection has over 375 plants of about 175 different kinds. Many are cultivars, or cultivated varieties, selected for horticultural merits such as flower size and color. Others are the parents of many of today’s hybrids.

Lilacs are in the olive family; other “relatives” growing on Bussey Hill include ash trees and forsythia.

The Arboretum is open from dawn to dusk, with refreshments and activities available from 10 a.m. to 4 p.m. Street parking along the Arboretum perimeter is limited, and visitors are advised to take public transportation. No unauthorized vehicles will be allowed within the gates during the event.

There’s an online brochure for a self-guided tour of the lilacs.