This article contains only basic information about Balts' pagan traditions. If you want to learn more - read some of books or visit websites that I recommend you at the end of this text.
Sources and historical background
Today exist only three baltic nations - Lithuanians, Latvians and Samogitians (living in Lithuania). But back in the Middle Ages they were much more numerous - there existed many bigger and lesser tribes, inter alia the nation of Old Prussians, the biggest and the most advanced from all of baltic people. All of those tribes had their own original culture and beliefs, closely related with slavic ones. Unfortunetely, they high skill of warfare was a big problem for christianized slavic and nordic rulers. There were organized many crusades against them, along with even more numerous peaceful missions, but most of them was a complete disaster. The Baltic people stood bravely against christians for many ages, but they were gradually forced to convert into new faith. Here you can read a bit about this topic; it's a link to blog of Peter of Skyforger: http://history.skyforger.lv/2011/crusades-against-pagan-northern-europe/
Since Baltic people were a last christianized Europeans, we have quite many information about their believes. The main source are of course christian chronicles ("history is written by victors") like e.g. Chronicon Prussiae by Peter Dusburg. Many information is also hidden in the folklore, and auxiliary sciences like comparative mythology and linguistics.
A baltic pantheon is really big, so here I will present only the most important gods and goddesses:
Dievas, Dievs - he is the supreme god of baltic pantheon, a ruler of the sky and shine, and also a king of all gods. His name means literally "God". Worshipped as a god-creator, he's a patron of sowing and a storm. Presenting with seed basket and a lightning in each hand. He's bounded with beneficial, positive powers of the universe. In myths bounded with a storms he's almost always relieved from the duties by Perkunas. Unlike other gods-creators, he have never became deus otiosus.
Perkunas, Perkuno - is a baltic god of thunder, lightning and rain. Closely related to slavic Perun, as well as nordic Thor. He is presented with many different weapons, mainly with axe. He is a creator of his weapons, sometimes with a helo of a god-blacksmith Televalis Representing positvive, active forces of the universe, Perkunas is a defender of the humanity. He's sacred tree is oak.
Velnias, Velinas, Vels, Vielona - the opponent of Perkunas. God of the underworld, death and dead people. In some way he is bounded with destructive magic.
Saule - is a goddess of the sun, a wife of Menu and a lover of Perkunas. He is presented as a women with a ewer, pouring out the light and warm after a long, cold winter. She represents a positive solar force.
Menu, Menulis - a god of the moon, and a husband of Saule. A tale says that his chlidren are stars.
Zemyna, Zeme Mate - is a Mother-Earth, responsible for the fertility and life of all beings. She is said to be a wife of Dievas (or sometimes Perkunas), with whom on spring solstice she inosculate again, and thanks to that gives another life to the earth (after a winter).
Laima - "a fate", or "a luck". She is the goddess of the human's destiny, appearing in most important moments of life. A patron of births and childhood. Sometimes she is also presented as a goddess of love (instead of nowadays goddess of love Milda).
Gabija, Gabia, Gabeta - "a candle", or "a light". She's a patron of the fire, linked with a sacred fire Aukuras. She's also a patron of household and a family.
Prusiauzemis - celebrated in January, is a festival of change of cosmos in winter.
On Ferburary there are celebrated several days: a day of Gabija, a day of Perkunas and the most important Uzgavenes. On this day is burnt down an effigy symbolising a bad, winter spirits.
Spring equinox - xelebrated on march.
Jore - a day of spring, devoted to Perkunas. On this day a thunder god awakes all nature to live.
The day of Milda - a day of lovers.
Rasos - a celebration of summer solstice in June.
Zoline - celebrated in August, is a festival of grass, cereal and a Mother-Earth Zemyna.
The day of Perkunas - celebrated on September, together with autumn equinox.
The day of all souls - festival lf the dead, celebrated in November.
Kucios, Kaledos and the day of Praamzius - the festivals bounded with winter and winter solstice.
M. Gambitas - "Balts. Ancient people and places"
V.R. Dzundzilla - "Balitc Lithuanian Religion and Romuva"
H. Biezais - "Baltic Religion"
For polish-speaking people:
J. Suchocki - "Mitologia Bałtyjska"
A. Zubiński - "Mitologia Bałtyjska"
J. Kierszka "Mity i Legendy Prusów"
O. Miłosz - "Legendy i podania litewskie"