It's a Jewish tradition to serve malt whisky at simchas and to drink this also on Purim (in addition to wine at the seuda).
From where does this custom derive?
From the passuk in tehillim, psalm 116
ובשם ה' אקרא אנא ה' מלטא נפשי
and I implored to Hashem - make my spirit a malt.
In fact we don't recite this part on the last days of Pesach because it might stress us too much that malt is chametz so we can't drink any. (I need to ask though why we don't skip it all of Pesach).
I guess they didn't want us to get too drunk on Rosh Chodesh either.
Since it is within 30 days before pesach and according to different sources one fan be mekayem tashbisu by eating, we drink the whiskey in order to get rid of it before pesach
As for other simchas I surmise that whiskey was easier to make versus wine in cold climates was more difficult to harvest grapes. Whiskey all you need is wheat and barrels (relative simplification) thus,what wine would gave been used for to celebrate whiskey came to replace it as the go to drink. Interestingly Alexander Chassidim still to this day make Kiddush on shnapps as per the minhag in Europe where wine was harder to procure
The Rabbis were concerned that if someone does not drink their whiskey before Pesach, they might pour the whiskey on the fire while burning chametz, which is an extremely dangerous thing to do. Yet because it is also a very fun thing to do, there was the concern that people would do it anyway, and get burned. Therefore, for reasons of pikuach nefesh, one must drink all of one's whiskey on Purim.
I have heard once from the Trenker Rebbe that the Roshei Teivos of מאלט spells out מ'שנכנס א'דר ל'שמחה ט'וב. Therfore we drink it on Purim and by other Simchas.
we find that the primary time of consumption of malt whiskey is at night (particularly at shalom zachors) this is because the gimatria of מאלט is 80 which the same gimatria of הלילה. Since הלילה has the particular article we know that it refers to night that would otherwise be understood. This is the night of a holiday or shalom zachor) when we already know the mitzva to drink as it says:
אין שמחה אלא בבשר ויין
(Not to imply that whiskey fulfills אין שמחה אלא בבשר ויין, rather אין שמחה אלא בבשר ויין tells us which night(s) the ה in הלילה refers to-nights on which we a have a general obligation to celebrate).
We drink malt whiskey in particular since whiskey (ויסקי) is gimatria כצלמו (in his image). At times that we are obligated to get drunk we remind ourselves that we are created in the image of God to make sure we act appropriately.