On what occasions do the population take soma in Brave New World?
We are given the answer to this question at the end of Chapter Three, in one of the snippets of quotes we hear from Mustapha Mond. Soma, it becomes clear, is a drug that has been created specifically to prevent the population of this world from thinking and becoming discontented with life or yearning for something more:
"... if ever by some unlucky chance such a crevice of time should yawn in the solid substance of their distractions, there is always soma, delicious soma, half a gramme for a half-holiday, a gramme for a weekend, two grammes for a trip to the gorgeous East, three for a dark eternity on the moon; returning whence they find themselves on the other side of the crevice, safe on the solid ground of daily labour and distraction..."
Soma, then, has been created for this world to act as a drug to stop people being worried or preoccupied by any dangerous or subversive thoughts about their lives or society which might make them want to do something out of the realm of the ordinary and therefore ruin this "perfect" society which guarantees everyone's happiness through drugs.