Q. Why do Chasidim drink whisky on a Yahrzeit?
A. First it must be said that although a Yahrzeit is a German term adopted by Jews in the late middle ages, the notion of commemorating the anniversary of a parent or teacher’s death was well known in the Talmud. The occasion was generally marked by fasting, though this has lapsed among many circles.
The saying of Kaddish for the first eleven months after death is regarded as helping the soul to enter Gan Eden; on a Yahrzeit, according to the great kabbalist Isaac Luria (the AriZal), Kaddish elevates the soul every year to a higher level in Gan Eden. Hence Chasidim celebrate the fact that year by year the soul has experienced a spiritual ascent.
A further reason which some give for a L’Chayyim on a Yahrzeit is that people feel depressed when they recall their loss and use spirits to raise their spirits.