The weather in Lithuania is temperate continental. In Vilnius the average highest daily temperature in July is 22,1 C whereas lowest average daily temperature in January is -8,7 C. Typically there are several very hot weeks in summertime and two very cold weeks in winter.
Water temperature in the Baltic Sea is around 18 C – 20 C in the summer. In lakes and rivers it gets significantly hotter. Lithuanian terrain is extremely flat, all country is under 300 m.
The time zone for Lithuania is UCT+2 in summer and UTC+3 in winter (due to daylight savings time).
All in all, there is no bad weather if you come for your party weekend in Vilnius, so bring a good mood!