Question: "When does the time change occur?"  Context: "Beginning and ending dates are roughly the reverse in the southern hemisphere. For example, mainland Chile observed DST from the second Saturday in October to the second Saturday in March, with transitions at 24:00 local time. The time difference between the United Kingdom and mainland Chile could therefore be five hours during the Northern summer, three hours during the Southern summer and four hours a few weeks per year because of mismatch of changing dates."  Answer:
second Saturday in October to the second Saturday in March, with transitions at 24:00 local time