Washington, where it's located

The climate of Washington, D.C. (District of Columbia), the US federal capital, is semi-continental, with cold winters, sometimes snowy, and hot and muggy summers. The city is located on the 38th parallel, somewhat halfway between between the cold and the warm part of the United States. In winter it can be affected by cold air masses coming from Canada, but less frequently than in New York; in summer the heat is felt, with long periods of sunshine and hot weather, but also with salutary afternoon or evening thunderstorms that come to temporarily break the heat.
Here are the average temperatures of the Reagan National Airport, a few kilometres (or miles) south of the city center, in Arlington County, Virginia; here a mild microclimate is found, because of the Potomac River, which widens becoming almost a lake, so lows are a couple of degrees Celsius (3.5 °F) higher than in peripheral districts and in the surrounding countryside.
Washington - Average temperatures
Min (°C)-2-13814192221171050
Max (°C)681319242931302620148
Min (°F)283037465766727063504132
Max (°F)434655667584888679685746

Precipitation in Washington is quite abundant, since it amounts to about 1,000 millimetres (40 inches) per year, and is evenly distributed over the seasons. In summer, the rains mainly occur because of afternoon or evening thunderstorms, as well as for the passage of some disturbances.
Here is the average precipitation.
Washington - Average precipitation

Winter, from December to February, is cold: the average temperature is just above freezing (0 °C or 32 °F). At night, temperatures are generally around freezing or slightly below, while during the day they typically exceeds freezing by a few degrees. The weather is unstable, so that sunny days alternate with rapid outbreaks of cold air, with snowfalls and temperatures around freezing; in these situations the wind can exacerbate the feeling of cold. In a year, an average of about 40 centimetres (15.5 in) of snow falls, while in Dulles Airport, actually located 37 km (23 mi) north-west of the city, in the state of Virginia, snowfall reaches 65 cm (26 in). Big snowstorms, which can block or hinder many activities, called Snowmageddon, occur on average every 4/5 years. During the most intense cold waves, in which the temperature remains below 0 °C (32 °F) even during the day, lows can plunge to -15 °C (5 °F), and sometimes even -20 °C (-4 °F).
However, there may also be some days, or periods of a few days, particularly mild, in which the temperature can reach 15 °C (59 °F), and sometimes even 20 °C (68 °F), which confirms the instability of the climate of Washington and in general of the American East Coast.

In spring, from March to May, the weather continues to be unstable: the first warm days alternate with the return of the cold. In March it can still snow, and sometimes this can also happen in early April. But already in April sometimes the temperature can exceed 25 °C (77 °F).
The National Cherry Blossom Festival lasts three weeks and starts on the last Sunday of March (therefore it ends in mid-April), when the cherry trees from Japan, donated at the beginning of the 20th century by the mayor of Tokyo, are in bloom.
By May, the average temperatures are pleasant, although there are still cool and rainy days, especially in the first half, while the first heat waves can bring the temperature above 30 °C (86 °F). On sunny days, the first afternoon thunderstorms can erupt as well.

Summer, from June to August, is hot and muggy, especially in July and August. There can be days with pleasant temperatures, around 25/27 °C (77/81 °F), maybe with some rain, but more often there are hot and sultry days, with highs around 30 °C (86 °F). There may also be heat waves, which can last a week, and in which the daytime temperature can reach or exceed 35 °C (95 °F), and the night temperature remains high as well, around 25/26 °C (77/79 °F). More rarely, as happened in July 2012, the temperature can reach 40 °C (104 °F). The sunshine amount in summer is good, although in fine weather thunderstorms can break out in the afternoon, in addition to the passage of some disturbances, which can bring clouds and rain for a few days.

Autumn, from September to November, is the mildest and the least unstable season of the year, at least in the first part. In the first half of September the weather is often still characterized by heat and humidity, with sunshine and some afternoon thunderstorms, then it becomes milder and relatively quiet until mid-October, while in the second half of October it can start to be a bit cold. November is a cold month, in which the first snowfalls can occur. The rains are not missing also in autumn, but this is still the season when the rains are a little less frequent.

Lincoln Memorial from above

The amount of sunshine in Washington 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 9.5 hours of sunshine per day in June, 9 in July, and 4/4.5 in December and January.
Washington - Sunshine

For swimming, the sea along the coast to the east of Washington, in the Chesapeake Bay (see Chesapeake Beach, Annapolis), about an hour's drive from the city, is a bit warmer than on the Atlantic Coast (see Dewey Beach, Ocean City), where in summer the temperature is around one degree Celsius (2 °F) lower.
Here is the average sea temperature in Atlantic City, on the Atlantic coast, a little north of Ocean City.
Atlantic City - Sea temperature
Temp (°C)6446111722232117139
Temp (°F)433939435263727370635548

When to go

The best periods to visit Washington are spring and autumn, to avoid both the winter cold and the summer heat, and in particular from mid-April to mid-May, and from mid-September to mid-October. In early September there's often still a sticky heat, though not as intense as in midsummer. In April, 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 trees losing their leaves (the phenomenon called fall foliage).