Map from Google - Czech_Republic

The climate of the Czech Republic is moderately continental, with cold winters (during which the temperature is often below freezing) and warm summers, during which the nights remain cool.
The majority of the Czech Republic is located at altitudes between 200 and 500 metres (600 and 1,600 feet), and has a fairly homogeneous climate. The capital Prague is distributed between 180 and 400 metres (600 and 1,300 feet).

Precipitation is fairly frequent, but it's not abundant in most of the country, around 400/500 millimetres (15/20 inches) per year; however, being scarcer in the winter months (when the vegetation goes to rest) and more abundant in late spring and summer, it's generally sufficient for agriculture. The only moderately rainy area are that of the Sudetes Range, where rainfall exceeds 1,000 mm (40 in) per year, that at the foot of the same mountain range (in Ostrava, precipitation exceeds 700 mm (27.5 in) per year), and that of the south-west, along the border with Germany and Austria.
Here is the average precipitation in the capital Prague, where 525 mm (20.5 in) of rain or snow per year fall.
Average precipitation - Prague
PragueJanFebMarAprMayJunJulAugSepOctNovDecYear
PragueJFMAMJJASONDY
Prec. (mm)252530407575657040303025525
Prec.(in)111.21.6332.62.81.61.21.2120.7
Days7667101099767790

Winter snowfalls are therefore frequent but generally not abundant. During the summer, on sunny days there can be thunderstorms in the afternoon, while Atlantic weather fronts, which can move over the country even in this season, may bring some cool and rainy days.
Here are the average temperatures of Prague.
Average temperatures - Prague
PragueJanFebMarAprMayJunJulAugSepOctNovDec
PragueJFMAMJJASOND
Min (°C)-5-4-137111212940-3
Max (°C)0381318212323191362
Min (°F)232530374552545448393227
Max (°F)323746556470737366554336

During winter, from December to February, in Prague and other Czech cities, the temperatures hover around freezing (0 °C or 32 °F) or just below, the weather is often dull and cloudy, and some light snowfall can occur. On the contrary, outbreaks of cold from Russia can bring frosty days, in which the temperature can drop to -20/-25 °C (-4/-13 °F).

In spring, from March to May, the weather is unstable, so that the first warm days alternate with the return of cold weather: frosts and snowfalls are still possible in April, especially in the first part of the month. A turning point is usually the second half of May, when the maximum temperatures regularly reach 18/20 °C (64/68 °F), and a stay in the country becomes enjoyable.

Summer, from June to August, is usually warm. There's no shortage of sunny days, during which some showers or thunderstorms in the afternoon may occur. Even in summer there can be cool and rainy days, with highs around 20 °C (68 °F), alternating with warmer periods, in which the temperature may exceed for a few days 30 °C (86 °F). However, we can generally expect pleasant temperatures, with highs around 22/23 °C (72/73 °F), and lows around 12/13 °C (54/55 °F).

Autumn, which goes from September to November, is initially pleasant in the first half of September, and becomes progressively more cloudy and cold, with some chance of rain.

Krumlov

In the north, in the mountainous area of the Sudetes, on the border with Poland, the temperatures decrease with altitude, especially in summer: at 800 metres (2,600 feet) above sea level, it is cool in summer, when the daytime temperature is typically around 18 °C (64 °F).
The highest point in the Czech Republic is Mount Snežka, 1,603 metres (5,259 feet) high. Here are the average temperatures of the top of Mount Praded, at 1,500 metres (5,000 feet) above sea level, in the Eastern Sudetes.
Average temperatures - Praded
PradedJanFebMarAprMayJunJulAugSepOctNovDec
PradedJFMAMJJASOND
Min (°C)-8-8-5-2257740-4-7
Max (°C)-4-4-11710111284-1-3
Min (°F)181823283641454539322519
Max (°F)252530344550525446393027

When to go

The best time to visit the Czech Republic runs from mid-May to mid-September, being the warmest or at least the mildest. The temperatures can be cool in May and September, and sometimes in June, while they are normally higher in mid-summer (July and August).

What to pack

In winter: warm clothes, sweater, down jacket, hat, gloves, scarf.
In summer: light clothes, t-shirts and shorts, but also long pants, jacket and sweatshirt for the evening or cooler days; raincoat or umbrella.