Map from Google - Valencia

In Valencia, a city located on the east coast of Spain, and in the Valencia Coast (which includes towns like Sagunto, Alboraya, Cullera, Gandia, Miramar, Piles and Oliva), the climate is Mediterranean, with mild and relatively rainy winters, and hot and sunny summers.
The average temperature of January is 10.5 °C, that of August 26 °C. Here are the average temperatures recorded at the airport.
Average temperatures - Valencia
ValenciaJanFebMarAprMayJunJulAugSepOctNovDec
ValenciaJFMAMJJASOND
Min (°C)5681013182121181496
Max (°C)161719212428313128241916

The rains amount to 470 millimeters per year, and follow a Mediterranean pattern, being concentrated from September to April. Even in the rainiest months, the rains are quite rare. The rainiest season is autumn, because of the prevalence of southerly winds; in winter it does not rain much, since the weather fronts come from the west, while the city is facing east. In summer, it almost never rains.
Here is the average precipitation.
Average precipitation - Valencia
ValenciaJanFebMarAprMayJunJulAugSepOctNovDecYear
ValenciaJFMAMJJASONDY
Prec. (mm)403530404018131765705550470
Days44355312554445

Winter, from December to February, is mild, and is characterized by sunny periods, alternating with periods of bad weather, with wind and rain, which however are quite rare. There are several sunny days, in which the daytime temperature reaches 17/20 degrees, even in winter. Very cold days are rare: only on colder nights the temperatures goes down to freezing or a few degrees below, but even in these cases, during the day they typically remain around 10 degrees. Although in inland areas of the Valencian Community it can sometimes snow at hill altitudes, snow in the city is a very rare phenomenon: the last major snowfall dates back to the 11th of January 1960; a less intense snowfall has been recorded in the 12th of February 1983.

Spring, from March to May, is mild and sunny, with rainy days still possible, but not very frequent, and afternoon thunderstorms which can stray from the interior in late spring. Starting in May, and sometimes in April, the first hot days occur, with highs around 30 °C.

Summer, from June to mid-September, is hot and sunny. Typically, temperatures hover around 28 degrees in June and September, and 30/32 °C in July and August, there's a humid heat, but also the breath of the breeze, in short, the typical Mediterranean summer. Sometimes, however, there may be heat waves, in which highs can reach 35 degrees. When the wind blows from the interior, temperatures can touch or exceed 40 °C, but fortunately in this case the air is dry.

Autumn, from mid-September to November, is initially warm, and then becomes gradually milder, while tre first Atlantic depressions begin to pass. Every now and then, the rains are concentrated in a short time and can be heavy.

The amount of sunshine in Valencia is very good in the summer, when clear skies prevail, it is good in spring and autumn, and decent even in winter: there are on average 5 hours of sunshine a day in December, 5.5 in January and 6 in February.

Valencia, Plaza del Ayuntamiento

The sea in Valencia is warm enough for swimming from July to September, when it reaches 24/26 degrees (ie, more than in other parts of Spain); it remains cool until June.
Here are the average sea temperatures.
Sea temperature - Valencia
ValenciaJanFebMarAprMayJunJulAugSepOctNovDec
ValenciaJFMAMJJASOND
Sea (°C)141314151721242624211815

When to go

The best time for a beach holiday in Valencia is the summer, from June to August. For swimming, the best time is from July to September, although from September, which however is still a good month (especially in the first half), the first Atlantic depressions begin to pass. May and June are sunny months, but the sea is still a bit cool, especially in May, when it is actually a bit cold.
Although good weather is not always guaranteed, spring and autumn are ideal for visiting the city and making excursions into the interior, as in summer it can be very hot.