Late spring through early autumn is the best window. May and September are particularly pleasant — warm days, cool evenings, and the surrounding farms in either planting or harvest mode. The summer months can be humid and hot, but the trees in the city's older neighbourhoods give plenty of shade.
Autumn brings the foliage in the surrounding hills and a busy stretch of harvest fairs in the county. Winters are cold and grey, but the city still works for a weekend visit; many of the indoor sights — the markets, Fulton Theatre, the museums — are at their best when the weather is bad.
First Fridays, the monthly gallery and downtown open house, are a good fixed event to time a visit around, year-round.