Boston, where it's located

In Boston, the capital of the State of Massachusetts, the climate is continental, with very cold, snowy winters and warm to hot summers, during which there are quite frequent thunderstorms. The city is located on the coast, but in the Atlantic Ocean, a cold current flows at this latitude, in addition, it's an area where different air masses collide: those coming from Canada and those coming from the Gulf of Mexico. The result is an unstable climate, with waves of bad weather alternating with clear and sunny days.
At the Logan International Airport, located right on the coast, the daily average temperature ranges from -1.5 °C (29.5 °F) in January to 23 °C (73 °F) in July. Here are the average temperatures.
Boston - Average temperatures
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, night temperatures are lower, as can be seen from the average temperatures of Brookline, located a few kilometers (or miles) from the coast, and south of Cambridge, across the Charles River. Here, the daily average temperature ranges from -2.5 °C (27.5 °F) in January to 22 °C (72 °F) in July.
Brookline - Average temperatures
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 millimeters (43 inches) per year, and it's well distributed over the seasons. In winter, it occurs mainly in the form of snow, while in summer, in addition to some weather disturbances that can bring some rainy days, thunderstorms in the afternoon or in the evening are quite frequent. Here is the average precipitation.
Boston - Average precipitation

Winter, from December to mid-March, is very cold: the average temperature is just below freezing (0 °C or 32 °F), but cold spells, with temperatures dropping to -15 °C (5 °F) or 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 centimeters (27 inches) 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, during which highs can reach 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 cold weather. In March, which is often still a wintery 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, the temperature can sometimes reach or even exceed 25 °C (77 °F). By May, the average temperature is pleasant, although there are still cool days, or even cold and rainy, while the first heat waves can raise 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 fine, in the afternoon, a cool and moderate breeze rises from the sea.
The Boston Marathon is held on the third Monday of April, ie around the middle of the month, when the average temperatures are very cool or even a little cold: in April, the 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 warm to hot and humid. Different situations can occur in this season as well: there are cool days, maybe with some rain, and with highs around 23/24 °C (73/75 °F), but also hot and quite muggy days, with highs around 28 °C (82 °F), with sunshine and the sea breeze. Sometimes, even heat waves can occur, with daytime temperatures reaching or exceeding 35 °C (95 °F), and night temperatures remaining around 23/25 °C (73/77 °F). The amount of sunshine during the summer is good, however, in addition to some weather fronts, there is a fair amount 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 it quickly becomes mild until mid-October, while in the second half of October, it can start to be a bit cold. November is already a cold month, and the first snowfalls can occur. Rainfall occurs quite often also in this season.

Boston from above

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. Here are the average sunshine hours per day.
Boston - Sunshine

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

Best Time

In order to avoid both the winter cold and the summer heat, the best times to visit Boston are spring and autumn, and in particular from mid-May to mid-June, and from early September to early October. June is often a good month too, but we can not exclude some hot days. In early September, the weather is often hot and humid, although not as hot as in midsummer. In May, it can sometimes 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 colors of the trees losing their leaves (the phenomenon called fall foliage).