Nearly all the year is comfortable for visiting Sochi as it belongs to that tiny part of Russia, which is happily located in the subtropical climatic zone. Sochi's summer usually lasts from the mid-end of May till the end of October. This is the true high season with its tourist peak in July-August, it also called 'swimming season'. The sea is warm and the air temperature is above 30°C (86°F).
In September and October the Black Sea is still warm, air is not very hot, and the city attracts fewer visitors. This period is called smoothy season ('barkhatny sezon' in Russian). Early spring and autumn are warm too, but with more cloudy days and rain. Daily average temperature even drops below 10°C (50°F) sometimes. These seasons can be really enjoyable for visiting Sochi.
But if you plan a winter trip, know the climate of the mountainous part of Big Sochi called Krasnaya Polyana is significantly colder. It allows a full ski season in winter (from December to March, and February is the best month). So don’t forget your warm clothes!