Just before Passover of 1657 in the town of Rozana Poland the body of murdered Christian boy was found in the courtyard of a Jewish home. The Christian community rose up in anger and claimed that the Jews needed the blood for making matzos. The court decreed that if the guilty person did not come forward then the entire Jewish community would be sentenced to death.
To spare the Jewish community the two Rabbis, R'Israel and R'Tuvia confessed to the murder. On the second day of Rosh Hashana, Friday the 9th of September 1659 (5420), the death sentence was carried out.
The sons of R'Israel took the surname Za'k which represents the word 'zera kodesh' meaning 'Holy Seed'. Later the name was changed to Zackheim which means 'They Are of the Holy Seed'
A more complete story can be found here: http://www.jewishgen.org/Yizkor/ruzhany/ruz010.html#Page18