Map from Google - Lithuania

Lithuania, a flat country overlooking the Baltic Sea, has a semi-continental climate, with cold winters, and mild and moderately rainy summers.

Winter is cold and dark, with temperatures below freezing (0 °C or 32 °F), and snow-covered landscapes from December to mid-March. The temperature decreases gradually as we proceed inland. In the coast (see Klaipeda), the average in January and February is around -5 degrees Celsius (23 °F) at night and around freezing during the day, while it decreases by a few degrees in the central area (see Kaunas), and even more in the east, so that Vilnius, the capital, is the coldest city in Lithuania, with a daily average of around -5 °C (23 °F) in January and February.
Snowfalls are frequent but not abundant, and often occur in the form of a continuous light snow. The temperature can change a lot depending on meteorological situation: when the currents come from the Atlantic Ocean, it can exceed freezing even in winter, while when they blow from Russia, it can plunge to -20 °C (-4 °F), or at times even -30 °C (-22 °F).

In spring, the temperature gradually increases, even though the weather remains cold for a few weeks; April is typically a cold month, and especially in the first half there's still some chance of snow and frost, while in the second half the first warm days are possible, brought by southerly winds. In general, the temperatures become pleasant in a stable manner from the middle of May.

Summer, from June to August, offers pleasant temperatures: lows hover on average around 11/13 °C (52/55 °F) across the country, while highs gradually increase, from 19 °C (66 °F) on the coast, to 22/23 °C (72/73 °F) in inland areas. In addition, summer has very long days, although it's a fairly rainy season.
Sometimes, warm currents from the southernmost plains of Eastern Europe can cause a considerable increase in temperature: in these cases, it can touch 25/30 °C on the coast, and even 30/35 °C (86/95 °F) in inland areas. However, these hot periods generally last only a few days.

Autumn is still quite pleasant in September, while starting from October it becomes dull and rainy, and gradually colder over the weeks. The first snowfalls typically occur on late October or in November.

Here are the average temperatures of Vilnius.
Average temperatures - Vilnius
VilniusJanFebMarAprMayJunJulAugSepOctNovDec
VilniusJFMAMJJASOND
Min (°C)-8-8-41710121183-1-5
Max (°C)-3-22101821222116103-1
Min (°F)181825344550545246373023
Max (°F)272836506470727061503730

Precipitation in Lithuania are frequent throughout the year, however, in terms of quantity there's a relative minimum between January and March (when it often occurs in the form of light snow), and a maximum in summer.
Here is the average precipitation in Vilnius.
Average precipitation - Vilnius
VilniusJanFebMarAprMayJunJulAugSepOctNovDecYear
VilniusJFMAMJJASONDY
Prec. (mm)403535455075807565505050655
Prec.(in)1.61.41.41.8233.132.622225.8
Days141210912991010101214131

Vilnius

The Baltic Sea is very cool even in summer: the water temperature goes from 2 °C (36 °F) in February and March, to 16 °C (61 °F) in July, and 18 °C (64 °F) in August.
Sea temperature - Klaipeda
KlaipedaJanFebMarAprMayJunJulAugSepOctNovDec
KlaipedaJFMAMJJASOND
Sea (°C)32247121618161295
Sea (°F)373636394554616461544841

When to go

The best period to visit Lithuania is summer, from June to August: the temperatures are pleasant, sometimes even warm. Certainly, there's no shortage of rainfall, often in the form of thunderstorms in the afternoon, but there are also some nice days. As it's often the case in Nordic countries, from mid-August the summer already begins to decline: the rains become more frequent, and in any case the days become shorter.
If you want to visit Lithuania in its winter appearance, February is preferable to December, because it has longer days.

What to pack

In winter: very warm clothes, down jacket, hat, gloves, scarf.
In summer: clothes for spring and autumn, being ready to add or remove the outer layer, t-shirt, but also long pants, jacket and sweatshirt; raincoat or umbrella.