Perun

Perun, the son of Svarog, is one of the most important Gods of the Slavic Pantheon (If not The most important, depending on who's asked!). He is a God of storms and war, he controls the thunder and lightning, and he is also the lord of the forests and mountains. He's a patron God for armies, kings and warriors, and he protects Russia from its enemies. Perun is represented as a man with a silver head and gold mustache, and it is said that eight flames accompany his images. His symbols include the sun, the axe, and a six spoked wheel called the Thunder Sign. An old tradition says that he gathers fallen warriors to fight in his army against demons.

Perun's Day (Chervan)

Perun's Day on July 20th was seen as a Terrible Holiday by many; on this day: Lots were cast on who would be the God's next sacrifice. The God required two sacrifices to keep him placated enough not to destroy people's crops: One sacrifice of a bull, another of a person. The bull would be cooked and shared in a feast among the people.

(from the Leszi Website) According to Dr. Buhler in De Diis Samogitarum, the prayer uttered by the officiating priest went as follows:
Perkons! Father! Thy children lead this faultless victim to thy altar. Bestow, O Father, they blessing on the plough and on the corn. May golden straw with great well-filled ears rise abundantly as rushes. Drive away all black haily clouds to the great moors, forests, and large deserts, where they will not frighten mankind; and give sunshine and rain, gentle falling rain, in order that the crops may thrive!"

The webmaster's note for celebrating this holiday; ixnay on the human sacrifice, hold a (beef!) barbeque instead. You may want to sacrifice something personal; for example, donate money to charity.