Honshu, map

In Tokyo, the Japanese capital, the climate is temperate, with fairly mild, sunny winters and hot, humid and rainy summers. Like the rest of Japan, the city is affected by the monsoon circulation: in winter, the northwest cold currents will prevail, while in summer, they will be replaced by hot and humid currents of tropical origin.
The city overlooks the sea, in the bay of the same name. Given the relatively low latitude (we are on the 35th parallel) and the protection given by the mountains (we are on the east side), in winter, the currents of Siberian origin are felt little, so there's a good number of sunny days, and it's usually not very cold. However, cold and windy periods, with highs around 5/7 °C (41/45 °F) or less and possible snowfalls, can not be ruled out. In any case, night frosts are rare and usually light.
In summer, there can be hot and muggy periods, with sunshine, highs around 35 °C (95 °F), tropical nights, and high humidity. In other periods, there's bad weather, with wind and rain, and sometimes (especially in June and July), nights are a bit cool. August is the hottest month, both because of the thermal inertia of the sea and because of the lower frequency of periods of bad weather.
Here are the average temperatures.
Tokyo - Average temperatures
Min (°C)124914182223201484
Max (°C)101014192326293127221612
Min (°F)343639485764727368574639
Max (°F)505057667379848881726154

In the western suburbs, the temperatures are a bit lower, both because of the greater distance from the sea and because of the lower urban heat island effect, as you can see from the temperature of Hanno, in Saitama Prefecture.
Hanno - Average temperatures
Min (°C)-2-11712162021181160
Max (°C)8912172224283025191511
Min (°F)283034455461687064524332
Max (°F)464854637275828677665952

Rainfall in Tokyo is abundant, in fact, it amounts to 1,500 millimeters (60 inches) per year. Winter is undoubtedly the driest season; the rainiest months are September and October, also because of the contribution given by typhoons. In August, there is a certain decrease in rain frequency: on average, among the summer months, it is the driest and sunniest as well as the hottest. Here is the average precipitation.
Tokyo - Average precipitation

As mentioned, from June to October (but especially from August to early October), Tokyo can be affected by typhoons, the tropical cyclones of South-East Asia, which mostly affect the southern part of the country, but can also arrive here. The most intense typhoons can bring strong winds, torrential rains and heavy damage.

The cherry blossoming (Sakura) in Tokyo takes place around March 30th: there are dedicated events in the streets and parks where you can admire them.
Less famous but still interesting is the plum blossoming, which announces the spring, and occurs in Tokyo roughly between February 10th and early March. In some gardens, there are dedicated festivals (ume matsuri), as happens in the Yushima Tenjin, near the university, or in the Yoshino Baigo of Ome, a city in the metropolitan area, located near Hanno, where the flowering occurs a little later (roughly, from February 20th to late March).

The amount of sunshine in Tokyo is good in winter (compared to the length of the day), while the sun is not so frequent in the other seasons because of disturbances that occur in spring and autumn, as well as of the summer monsoon. In summer, there are only 4 hours of sunshine per day in June, 4 hours and a half in July and 5 hours and a half in August. Here are the average sunshine hours per day.
Tokyo - Sunshine


The sea is warm enough for swimming, with a temperature of 23 °C (73 °F) or more, from July to October, as you can see in the following table.
Tokyo - Sea temperature
Temp (°C)171616172022242625232119
Temp (°F)636161636872757977737066