Map from Google - Boston

In Boston, the capital of the State of Massachusetts, the climate is continental, with very cold and snowy winters, and warm (and sometimes hot) summers, with quite frequent thunderstorms. The city is on the coast, but in the Atlantic Ocean at this latitude a cold current flows, also it's an area where different air masses collide: the cold ones coming down from Canada and the warm ones from the Gulf of Mexico. The result is an unstable climate, with waves of bad weather alternating with clear and sunny days.
In the Logan International Airport, located right on the coast, the average temperature of January is -1.5 °C (29.5 °F), that of July is 23 °C (73 °F). Here are the average temperatures.
Average temperatures - Boston
BostonJanFebMarAprMayJunJulAugSepOctNovDec
BostonJFMAMJJASOND
Min (°C)-5-4-15101519181483-2
Max (°C)24713192427262216115
Min (°F)232530415059666457463728
Max (°F)363945556675817972615241

The airport is in the mildest area of the city; in inland neighborhoods and suburbs the night temperatures are lower, as can be seen from the average temperatures of Brookline, just a few kilometres (or miles) from the coast, and south of Cambridge, across the Charles River. Here, the average temperature of January is -2.5 °C (27.5 °F), that of July is 22 °C (72 °F).
Average temperatures - Brookline
BrooklineJanFebMarAprMayJunJulAugSepOctNovDec
BrooklineJFMAMJJASOND
Min (°C)-7-7-2281316151161-4
Max (°C)22714202528272317104
Min (°F)191928364655615952433425
Max (°F)363645576877828173635039

Precipitation in Boston is quite abundant, since it amounts to 1,100 millimetres (43 inches) per year, and is well distributed over the seasons. In winter it occurs mainly in the form of snow, while in summer, in addition to the passage of some disturbances that can bring some rainy days, thunderstorms in the afternoon or in the evening are quite frequent. Average precipitation - Boston
BostonJanFebMarAprMayJunJulAugSepOctNovDecYear
BostonJFMAMJJASONDY
Prec. (mm)8585110959095858585100100951110
Prec. (in)3.33.34.33.73.53.73.33.33.33.93.93.743.7
Days111012111211109991112126

Winter, from December to mid-March, is very cold: the average temperature is just below freezing (0 °C or 32 °F), but cold spells, in which temperatures can drop to -15 °C (5 °F) and even below, are not uncommon. The freezing wind, called blizzard, can heighten the feeling of cold.
Snowfalls are frequent, and sometimes can be abundant, resulting in real snowstorms: for example during the storm of the 17/18th of February 2003, 70 centimetres (27 in) of snow fell, while on the 1st of April 1997, 64.5 cm (25.4 in) fell, and on the 26/27th of January 2015, 62.5 cm (24.6 in). Snow covers the ground for weeks. In a year, an average of 105 cm (41.5 in) of snow falls at the airport, and 115 cm (45 in) in the western districts.
On other occasions, the south wind can bring some mild days, with highs around 12/15 °C (54/59 °F), but this occurs less frequently than in New York.

In spring, from mid-March to mid-June, the weather continues to be unstable: the first warm days alternate with the return of the cold weather. In March, which in fact is often still a winter month, it snows on a frequent basis, and sometimes this can happen even in April, especially at the beginning of the month. But already in April sometimes the temperature can touch or even exceed 25 °C (77 °F). In May, by now the average temperatures are pleasant, although there are cool days, or even cold and rainy, while the first heat waves can bring the temperature above 30 °C (86 °F). On sunny days, also the first afternoon thunderstorms can erupt. In late spring and early summer, fog may form, maybe on rainy days. When the weather is nice, in the afternoon a cool and moderate breeze rises from the sea.
The Boston Marathon is held on the third Monday of April, therefore around the middle of the month, when average temperatures are very cool or even a little cold (the April average is 5 °C (41 °F) as minimum temperature and 13 °C (55.5 °F) as maximum temperature).

Summer, from mid-June to August, is hot and humid. Even in this season, different situations alternate: there are cool days, maybe with some rain, and with highs around 23/24 °C (73/75 °F), but also hot and a bit muggy days, with highs around 28 °C (82 °F), with sunshine and the sea breeze, while sometimes even heat waves can occur, in which the daytime temperature can reach and exceed 35 °C (95 °F), and even the minimum temperatures remains high, around 23/25 °C (73/77 °F). The sunshine amount during the summer is good, though there's no shortage, in addition to the passage of some disturbances, of afternoon thunderstorms, sometimes intense and accompanied by hail.

Autumn, from September to November, is the mildest and less unstable season of the year, at least in the first part. In early September, the weather can still be characterized by heat and humidity, with sunshine and some afternoon thunderstorms, then quickly becomes mild until mid-October, while in the second half of October it can start to be a little cold. November is already a cold month, and the first snowfalls can occur. There's no shortage of rainfall also in this season.

The amount of sunshine in Boston is good in summer, but it is not scarce even in winter, because as mentioned, some sunny days can always occur. On average, there are 9.7 hours of sunshine per day in June and July, and 4.5 in December.

Boston from above

The sea in Boston, very cold in winter, stays very cool even in summer, given that it reaches 18 °C (64 °F) in August. As mentioned, in the sea a cold current flows: in fact the Gulf of Maine in winter is affected by an offshoot of the Labrador current.
Here are the average sea temperatures.
Sea temperature - Boston
BostonJanFebMarAprMayJunJulAugSepOctNovDec
BostonJFMAMJJASOND
Sea (°C)543581317181613107
Sea (°F)413937414655636461555045

When to go

The best times to visit Boston are spring and autumn, to avoid both the winter cold and the summer heat, and in particular from mid-May to mid-June, and from early September to early October. June also is often a good month, but we can not exclude hot days. In early September a warm and humid weather often reigns, although not as intense as in midsummer. In May, sometimes it can get cold and the weather can be unstable, especially at the beginning of the month, while generally the second half of the month is mild.
In autumn (usually around late October) you can admire the beautiful colours of the trees shedding leaves (the phenomenon called fall foliage).