Lexington is a really lovely town and there's almost no bad time to visit, but there are definitely seasons here -- so schedule your  activities accordingly.  Here's a guide to help with your plans:

Spring -- April to June*

Characteristics:  Sometimes spring is non-existent and we go right from winter to summer.  Other times it rains for weeks on end. Occasionally we are rewarded for our patience through the long winter with a few absolutely gorgeous days.  Time your visit for those days if possible.  If you hit our "monsoon season,"be grateful for the chance to visit all of the fine museums in the area.  

Activities:  Revolutionary War buffs will enjoy the re-enactment that happens on the Battle Green every Patriot's Day (arrive early in the morning for a good view and follow the crowds to a pancake breakfast afterward).
 

Summer -- June to September

Characteristics:  The local weather forecasters like to use "triple-H" to describe our summer weather:  hazy, hot, and humid.  Thunderstorms are common during the afternoons and evenings when it's particularly humid.

Activities:  On steamy nights, drive out to Westford, MA to Kimball Farm to cool off with huge quantities of fresh ice cream.  Then burn off a few of those calories on the mini-golf courses or the bumper boats. 
 

Fall -- September to November

Characteristics:  Crisp cool air and colorful leaves on the trees.  Who could ask for anything more?  The timing of the "peak" foliage color varies from year to year, but usually happens in October.

Activities:  In the weeks before Halloween, head a few minutes west on Route 2 to Arena Farms in Concord, MA to see the giant pumpkins for sale and enjoy a fresh-dipped caramel apple.
 

Winter -- December to March
 
Characteristics:  Brrr!  Dress warmly and bring snowproof boots.  Some years we get clobbered with the white stuff, some years we get hardly any, but it's best to be prepared. 

Activities:   Go cross-country skiing on the bike path that runs through town, participate in the Messiah Sing held in mid-December, or just hunker down near the brick ovens at Bertucci's to thaw out and complain about the cold with everyone else.
 

* Warning -- timing of seasons highly subject to change!