Map from Google - Lithuania

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

On the coast the average temperatures are just below freezing in January and February, and around 18 °C (64 °F) in July and August. Here is the average temperature in Klaipeda.
Average temperatures - Klaipeda
KlaipedaJanFebMarAprMayJunJulAugSepOctNovDec
KlaipedaJFMAMJJASOND
Min (°C)-3-4-1371114141062-2
Max (°C)1141016182121171263
Min (°F)272530374552575750433628
Max (°F)343439506164707063544337

In inland areas, the temperatures are a bit lower in winter and a bit higher in summer. Here are the average temperatures in the capital Vilnius.
Average temperatures - Vilnius
VilniusJanFebMarAprMayJunJulAugSepOctNovDec
VilniusJFMAMJJASOND
Min (°C)-7-7-32711131284-1-5
Max (°C)-2-14121821232217103-1
Min (°F)191927364552555446393023
Max (°F)283039546470737263503730

Precipitation in Lithuania are frequent throughout the year, however, in terms of quantity there's a relative minimum between February and April. The coast is rainier than the interior: in Klaipeda precipitation amounts to 735 millimetres (29 inches) per year, with a maximum in autumn, while in Vilnius it amounts to 685 mm (27 in), with a maximum in summer (due to afternoon thunderstorms).
Here is the average precipitation in Vilnius.
Average precipitation - Vilnius
VilniusJanFebMarAprMayJunJulAugSepOctNovDecYear
VilniusJFMAMJJASONDY
Prec. (mm)504040405580857065555055685
Prec.(in)21.61.61.62.23.13.32.82.62.222.227
Days12101091011111010101012125

Winter is cold and dark, with temperatures below freezing (0 °C or 32 °F), and landscapes often snow-covered from December to mid-March. The temperature decreases gradually as we proceed inland: on the coast (see Klaipeda), the daily average in January and February is around -1 degrees Celsius (30 °F), while it drops by a few degrees in the central area (see Kaunas), and even more in the east, where it reaches -4 °C (25 °C) in Vilnius, which therefore is the coldest city in Lithuania.
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, as well as very long days, pleasant temperatures: highs gradually increase, from 18/21 °C (64/70 °F) on the coast, to 21/23 °C (70/73 °F) in inland areas. However, summer is 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 by October it becomes dull and rainy, and gradually colder over the weeks. The first snowfalls typically occur on late October or in November.

Vilnius

The sun in Lithuania is rarely seen from November to February, while from May to August it shines for a decent amount of time (the days are very long, so the total sunshine is not low).
Sunshine - Vilnius
VilniusJanFebMarAprMayJunJulAugSepOctNovDec
VilniusJFMAMJJASOND
Sun (hours)134588775311

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.