The high season and the best time to go is in May or September, when weather is mild and crowds fewer. Many museums, galleries, castles and the like are only open at this time. April and October are chillier but you’ll benefit from smaller crowds and cheaper rooms. In July and August hostels are chock-a-block with students, especially in Prague. In winter, you’ll likely get to see it all under a blanket of snow; camping grounds are closed, as are attractions in smaller towns but the time is right for skiing and other winter pursuits.
Most Czechs and Slovaks, like the rest. of Europe, take their holidays in July and August, and then again over the Easter and Christmas-New Year holiday period. Accommodation facilities are often booked; crowds, particularly in Prague and the mountain resort areas, can be unbearable; and prices spike to their highest. On the other hand, most festivals take place during summer months and the supply of cheap sleeps in university towns increases as student dorms are thrown open to visitors. High in the mountains, November through March is an additional high season.
Czech seasons are distinct. Summer (June through August), receives the highest temperatures and also the heaviest rainfall. The cold, bitter winter months of December, January and February often have temperatures reaching as low as -5°C (23°F) in the cities and -10°C (14°F) to -15°C (5°F) or even -30°C (-22°F) in the mountainous areas. They are tailor-made for skiing and other winter pursuits; the mountains receive about 130 days of snow a year, but other areas get coverage as well. Spring (late March to May) brings changeable, rainy weather and sometimes flooding. Autumn is also variable but temperatures can be as high as 20°C (68°F) in September.