Honshu, map

In Tokyo, the Japanese capital, the climate is temperate, with fairly mild and 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.
Tokyo overlooks the sea, in the bay of the same name. Given the relatively low latitude (we are on the 35th parallel) and the protection of the mountains (we are on the east side), in winter the currents of Siberian origin are felt little, so there are several sunny days, and usually it's 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. Anyway, night frosts are rare and usually light.
In summer, hot and muggy periods, with sunshine, highs around 35 °C (95 °F), tropical nights and high humidity, alternate with periods of bad weather, with wind and rain, and sometimes (especially in June and July) nights 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
MonthJanFebMarAprMayJunJulAugSepOctNovDec
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 lesser urban heat island effect, as you can see from the temperature of Hanno, in Saitama Prefecture.
Hanno - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)-2-11712162021181160
Max (°C)8912172224283025191511
Min (°F)283034455461687064524332
Max (°F)464854637275828677665952

The rains in Tokyo are abundant, in fact they amount 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
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)505512012514017015517021020095501530
Prec.(in)22.24.74.95.56.76.16.78.37.93.7260.2
Days5610101011108111074101

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 the 30th of March: there are dedicated events in streets and parks where you can admire them.
Less famous but still interesting is the plum blossoming, which announces the spring, and in Tokyo occurs roughly between February 10th and early March. In some gardens, there are festivals dedicated (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 the 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 hours in July and 5 and a half in August.
Tokyo - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours665654554456

Tokyo

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
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)171616172022242625232119
Temp (°F)636161636872757977737066